Ahimsa, or "non-harming," is fundamental to the ethics of Buddhism, as well as to the ethics of most Eastern philosophical/religious/spiritual traditions. It may, for all I know, be fundamental to spirituality or "Dharma" in general. Some people think that Buddhism goes a little overboard in this direction of nonviolence, however, and even many professing Buddhists continue to kill animals, especially small, pesky ones. I've seen Burmese laypeople, at monasteries yet, go into a berserk killing frenzy upon seeing some harmless, non-venomous snake. But then again, some take non-killing very seriously, even refusing to eat the eggs of their own chickens because there is a virile rooster strutting about, ensuring that the eggs contain live chicken embryos. I know a devout Buddhist lady in northwest Burma whose family were utterly scandalized when her brother married the daughter of a fisherman, a killer of animals.
On the other hand, interestingly, most Western religious traditions condone the killing of animals, or may even require it. I was surprised to read that at the Trappist monastery where Thomas Merton lived, the monks actually slaughtered their own livestock, and had a storeroom where bloody goat hides were stored. According to the Christian Bible, Jesus Himself, after he died, helped his disciples to catch a prodigious boatload of fish. Many people do not realize that Islam to this day requires bloody animal sacrifices—a pilgrim performing the Hajj at Mecca is expected to cut an animal's throat there. Traditionally the victim is to be a camel, although nowadays a goat is usually killed instead; I imagine that camels are rather expensive. Although Jewish animal sacrifices pretty much came to an end with the destruction of the Temple in 70 C.E., still rabbis work at slaughterhouses for the purpose of producing kosher meat. And of course, Judaism and Islam in particular condone even the killing of humans under special circumstances. Moses and Muhammad were both military leaders, and commanded the deaths not only of enemy combatants but of unarmed civilians. Moses commanded the complete extermination of the Canaanites, which, ironically, may be the first case of attempted genocidal ethnic cleansing in recorded history; and Muhammad at the very least commanded the massacre of a tribe of uncooperative Jews in Medina. (It may be though, now that I think of it, that most of the women were not killed, but just enslaved. Incidentally, the first time I read the Old Testament, when I reached Numbers 31:13-18, (click) a passage following shortly after Moses is called "the gentlest of men," I was sorely tempted to fling the Bible across the room in disgust.)
But even some forms of Buddhism and Hinduism allow the killing of conscious beings. For example, animals are regularly sacrificed to Kali in India; and I actually saw in an Indian edition of Where There Is No Doctor a stern warning against sacrificing human babies to Kali for the purpose of inducing fertility. So Western religion has no absolute monopoly on slaughtering and butchering.
Plenty of killing was going on in the Ganges valley in the Buddha's time too, of various sorts. The Brahmanisitic proto-Hindus still ate meat in those days, and killed vermin; there were frequent and bloody wars; there were public executions of criminals, in various nasty ways, for the good of society; and also there were Brahmin-conducted animal sacrifices not so different from those conducted by the contemporary paganistic Greeks. The Buddhist texts indicate that, although there was a wide variety of killing being practiced, the Buddha protested most outspokenly against animal sacrifice, possibly because he realized that Indian society was ripe for leaving it behind, and could accept what he said on the issue (whereas it was hardly likely that people were ready to stop eating meat then, or executing criminals, or waging wars). Or it may be that he was so outspoken against animal sacrifice because it was ostensibly a religious act, and he wanted formal religion, at least, to be ethically pure. Politics, as opposed to spirituality, tends to be beyond the direct influence of sages. However, Buddhist ideas of ahimsa did ultimately inspire many or possibly most Indians a few centuries later to be vegetarian, and even to avoid onions and garlic. Also Buddhist ideas inspired that great political anomaly Asoka. But although the Buddha apparently was reconciled to living in a violent age, the killing of conscious beings has always been forbidden to Buddhists, whether ordained renunciants or laypeople.
Abstaining from pāṇātipāta is the first moral precept for Buddhist laypeople, pāṇa meaning "breathing" and atipāta meaning something like "falling upon" or "attack." More specifically and less literally, it refers to killing macroscopic animals, including human ones. The killing of microscopic organisms, fortunately, is allowed; and laypeople are allowed to kill plants also, although ordained monks and nuns are forbidden even to damage a living plant deliberately—just detaching a single green leaf from a tree entails an ecclesiastical offense requiring expiatory confession. This may seem strict to the point of making life unnecessarily inconvenient (for instance, monks are forbidden to pull weeds or mow lawns at their own monasteries), but there are special allowances. For example, a monk is allowed to wipe mildew off a wall, thereby killing the poor mildew; he is allowed to set a water pot turning green with algae out in the sun to eliminate/exterminate the algae; and he may also spread bedbug-infested bedding in the hot sunshine to expel the bedbugs—thereby indirectly condemning most or all of them to an untimely death. I've taken advantage of that last allowance, and it turned into a massacre, with swarming ants capturing hapless bedbugs and, flushed with victory, carrying them on a bustling little ant highway back to their nest.
The Jains are rather more severe than are the Buddhists with regard to ahimsa; for example some of them sweep the ground before them with a soft broom in order not to step on tiny insects, and wear face masks like a surgeon in order not to inhale them accidentally. This is because the Jains consider killing in and of itself to be ethically impure, even if it is accidental or totally unperceived. On the other hand, Buddhist philosophy asserts that, as with all moral issues, killing is unethical or akusala (literally "unskillful") only if it is deliberate, or volitional. It is primarily the anger, hatred, or malice of the killer that constitutes the unskillful karma, not the suffering or death of the killed. Buddhist ethics is emphatically psychological, subjective, and volition-oriented. So stepping on a bug accidentally, possibly without even realizing it, is not unethical for a philosophical Buddhist. (The careless semiconsciousness which allows the person to walk around cluelessly squishing bugs may be unethical, however.) A monk who knows that he is killing animals, or that he will kill them, yet whose purpose is not to kill, also commits no offense. This presumably would apply to lay ethics with the first precept also. If a monk wants to walk from point A to point B, yet there are countless myriads of tiny, swarming insects covering the ground between these two points, then so long as he's not going to point B in order to step on them, his going is grounds for no offense, regardless of how many hundreds of them he steps on. I would assume that taking vermifuge (dewormer) medicine would be a similar case. (Some monks take care to use only dewormers that don't kill the worms, but only cause them to move out because they're temporarily paralyzed or don't like the food; but obviously, once the worm has left the protective environment of the monk's intestines, it's pretty much a goner anyway.) Refusing to feed a starving dog would also be no offense—or for that matter, watching somebody drown without lifting a finger to help—regardless of how cold-hearted and assholish it may be. Also, "striking out in self defense" is allowable to a monk, although not with intent to kill. But if he did accidentally kill some assailant while fighting for his life, he would not be considered guilty of murder. Thus monks in such situations must examine their own volitions very carefully.
Which leads to a consideration of homicide. For a layperson it would be included under the same first precept which forbids the salting of slugs, but for a monk it is the third pārājika, and grounds for excommunication from the Sangha for life, being the most grievous ecclesiastical offense a monk can commit—only deliberately lying about having attained superhuman mental states is possibly worse, that being the grounds for the fourth pārājika. Murder must be deliberate to qualify, and includes hiring an assassin, setting mantraps, killing by means of black magic, persuading somebody to commit suicide, and sponsoring or facilitating an abortion. (According to Theravada Buddhist philosophy, "rebirth-linking consciousness," and thus existence as a human being, begins at conception. And even if a person disagrees with that philosophy, as well one might, the Pali specifically mentions the case of abortion, and condemns it, and the rules remain the same.) There is actually a case mentioned in the Pali of a monk who exhorted an official executioner to keep his axe well sharpened and to aim carefully, in order to behead his clients as swiftly, painlessly, and mercifully as possible, and the executioner took his advice—resulting in that monk being judged by the Buddha to be guilty of murder, since he had instructed the executioner on how to kill. He was an accomplice. Buddhist ethics, and Buddhist ecclesiastical law, may be very subtle.
There are some strange gaping loopholes in the rules, however. For instance, a monk could hire an assassin and tell him, "Kill So-and-so at precisely 5:30pm," knowing full well that this particular assassin is always running behind schedule. So when the assassin finally gets around to murdering So-and-so at 6:02, the naughty monk is not guilty of pārājika #3 because the assassin did not act according to his instructions. I'm not exactly sure about this next one, but I'm pretty sure that if a monk tries to kill person X, but accidentally kills person Y instead, again he is not guilty of murder, at least according to Vinaya, because the only person he killed, he killed accidentally. (When I was a young Vinaya student I used to think about this stuff a lot.) For example he aims his assault rifle at X, but misses and hits Y. Or let's say he poisons a piece of cake, and gives it to a monastery attendant saying, "My good fellow, please give this cake to X, with my regards"; but then the greedy attendant, liking the looks and smell of that cake, eats it himself, and dies. The murderous monk killed the attendant completely unintentionally, and so is off the hook, and still a monk in good standing. He still makes seriously unskillful karma through his murderous intentions and efforts though, and is guilty of a lesser offense for trying and failing, despite his non-excommunicated status. I'm pretty sure. The moral of the story is: If you're a monastery attendant and are requested by a monk to give some food to another monk, don't eat it yourself.
With regard to monks killing animals other than humans, it is in violation of exactly the same rule, pācittiya #61—"And whatever bhikkhu intentionally deprives an animal (pāṇa) of life: pācittiya"—regardless of whether the animal in question is a mosquito or an elephant. This is the official ecclesiastical ruling, although the unskillful karma of the act is not necessarily equal in both cases. Interestingly, the official position that I have been taught is that the more volitional effort required in the killing, the more unskillful the act; and thus killing an elephant is worse than killing a mosquito because it requires more volitional effort, especially if one is trying to slap it to death. But it seems to me that killing an elephant could conceivably involve less effort than killing a mosquito. For instance, simply firing a shot with a high-powered rifle may be easier than exasperatedly chasing, with intent to kill, one pesky little mosquito (hard to see, but you can hear it whining) back and forth through one's dwelling before finally squishing the little bastard with a roar of triumph. I suspect that even in that case, killing the elephant could be more unskillful.
Following is a modern, biological consideration with regard to the killing of animals, which occurred to me as a university student and later as a professional biologist. Many invertebrates, such as mollusks and arthropods, lack a true central nervous system, consequently have a sensory system very different from ours, and apparently do not feel pain or fear. I was told by a monk with a degree in Neurophysiological Psychology, or some such, that they have no pain receptors at all. Thus they experience life much differently than we vertebrates do. (And by the way, the word "neurophysiological" contains all of the vowels.) One example based on my own experience arose when I was working as a biologist on a Japanese longline vessel in the Bering Sea: The ship caught bottom fish by laying about thirty kilometers of line, with baited hooks about every one meter, every morning, and spent the rest of the day reeling it in. One of my duties was to examine every crab that came on board, recording species, sex, size, weight, and viability—i.e., how healthy it was after being caught. The longline ran through a roller apparatus called a "crucifier" as it entered the ship, and crabs would often be crushed and mangled if they went through this thing. Anyway, all crabs were tossed into a holding tank and were attended to when I had the time. One time a crab came up and was tossed into the tank with most of its legs ripped off or totally mangled; it had six or seven of its ten legs destroyed. But, to my surprise, when it found a piece of squid bait, it calmly began eating it, apparently totally unconcerned that most of its legs had just been ripped off. (If a rat or mouse were to have three of its legs torn off and then a peanut placed before it, it certainly would not behave like that crab: it would in all likelihood go into convulsions and die.) Another example: An octopus, which is a mollusk, albeit a relatively extremely intelligent one (being, I have read, approximately as intelligent as a cat), may sit in its water and nervously bite off the ends of its own tentacles, apparently not bothered at all by any discomfort. And as for insects, I won't even tell you what happens if one severs the head of a male praying mantis. The mantis actually seems to like it. Thus some invertebrates, insects for example, may be essentially organic robots, with some kind of consciousness, but with no real perceptions, and no pain or fear. Thus a mosquito fleeing a slapping hand is no more afraid than the automatic door at the grocery store when it moves out of your way. Its body's circuitry is simply programmed to make it do that. I'm not trying to justify killing insects, crabs, or octopi here, but if the ethics of killing at all involves the suffering inflicted upon others, or the suffering added to the world in general, and not just the intention and effort required to perpetrate the slaying, then eating clams and shrimp may be a more compassionate and more ethical way of obtaining animal protein than eating fellow vertebrates like fish, let alone chickens, pigs, and cows.
Rather than justifying the killing of animals for food, or parasitic insects (because they're parasitic), or even plants, it is probably better to consider ALL life to be sacred—not just our own life, not just human life, not just the life of animals that we approve of or consider to be intelligent, but all of it, including bloodsucking, disease-causing parasites. But then why not consider everything else to be sacred too, like bricks, pop bottles, sin, and death? Life is sacred, death is sacred, everything is sacred. Then we have to kill what is sacred in order to live, but that's sacred too. To see and experience the sacredness of every being, of everything, and of every act would eliminate most, if not all, of our hangups and moral problems. See everything as Divinity, and then do as you please.
With regard to the question of whether the ethics of killing is purely subjective, i.e. depending entirely upon our own conscious volitions and not upon the feelings of the being attacked (not to speak of the will of an almighty God who determines ethics by decree), it is interesting that some kind of subconscious telepathy, or empathy, or anyway something beyond subjective, conscious volition appears to be involved, at least sometimes, even in the orthodox, official version of Theravadin Dhamma. One striking example is that, according to the authorities, killing an enlightened being necessarily always causes the perpetrator to go straight to hell (do not pass Go, do not collect $200) at the moment of death. To kill an arahant accidentally does not necessitate hell; but so long as the intent to kill is there, all else, including the amount of effort required, seems to be irrelevant—and this is presumably regardless of whether one even realizes that the being one is killing is in fact an arahant. How could a murderer, or any other unenlightened person for that matter, really know that anyway?
Following are a few more strange examples of karma involving enlightened beings. In the commentaries there is a story of a man who happened to see an enlightened monk adjusting his robes outside of a town. This monk had beautiful, shining golden skin, which showed during the adjustments, and the man was sexually aroused and thought, essentially, "Wow, I wish he were my wife, or that my wife had skin that looked like that." As a result of this lustful feeling and profane thought concerning a perfected saint, despite the plain fact that he had no idea that the monk was fully enlightened, he instantaneously (←also contains all the vowels) metamorphosed into a woman, and remained a woman until he apologized to the saint several years later. (Incidentally, this kind of instant sex change allegedly happened every now and then in the Buddha's time. Consequently, in some orthodox circles the spontaneous transformation of at least five ordained bhikkhus into women would be the only way to revive the order of fully ordained nuns in the Theravada Buddhist Sangha, since an instantaneous change would not invalidate the ordination like a gradual change, passing through the stage of a eunuch, would do. Eunuchs may not be monks or nuns. The whole commentarial story is a rather interesting one, and perhaps worthy of being translated on this blog someday.)
There is another story in the commentaries of a rich miser who offered food alms to a paccekabuddha (a fully enlightened being who lives during an interval when Buddha-Dharma has disappeared from the world), but shortly afterwards regretted it, considering that he had wasted it by giving it to a beggar, and that it would have been better just to have given it to one of his servants instead. As a result of his regret and contempt for the paccekabuddha, an omniscient saint, he went to hell for many eons after he died; but after finishing his time in hell, he went to heaven for more eons for having made the offering in the first place. (Or maybe he went to heaven first—I don't remember now—but the point is the same.) He could hardly have known that the beggar was a paccekabuddha, yet his momentary actions had tremendous results, seemingly far beyond the scope of his brief little volitions.
So in cases like this there is apparently something at work other than the mere conscious volition of the agent. I suspect it may be like this not only with arahants, but even with animals, rendering the killing an elephant or a chimpanzee or a sperm whale much worse than killing a mosquito, even though one might kill the one with a mere word or the push of a button, and might kill the other after half an hour of chasing it, slapping at it again and again, and angrily cussing like a sailor the whole time. For similar reasons I consider believing that one has killed some person or animal, but being mistaken, to be less weighty karma than having actually killed that person or animal, even though the subjective volitions and perceptions may seem exactly the same. There is, I believe, a deep connection of spirit between beings, and even the average person experiences this sometimes, maybe much more than she/he realizes, or is willing to believe. Maybe even all the time.
Long ago I asked my first Vinaya teacher, Which is more unskilful, performing an unskilful act, like killing, knowing that it's unskillful, or performing it without knowing? (I think what I had in mind at the time was people like terrorists who believe that their killing is not only good, but will actually get them into heaven, or, on a much smaller scale, people who consider hunting and fishing to be good clean fun.) My teacher said that it is worse to do wrong not knowing that it's wrong; and the example he gave was to the effect that, if you know something is hot before you handle it, then you will naturally handle it more carefully. So a person who kills while knowing that it is unethical, or bad karma, or wrong, is likely to be burned less severely than someone who thinks killing is glorious and honorable, or just a job. This makes sense; it seems fairly certain that any action performed with knowledge would be closer to enlightenment than one performed in ignorance. However, knowledge should not be confused with mere dogmatic belief, which is quite a different thing, and frankly, most people cannot tell the difference. Also the knowledge, or the mere belief, that one's action was unskillful may lead to subsequent feelings of guilt and remorse, which in themselves are unskillful, and make the situation worse than it already is. But is being positively glad that we acted unskillfully better or worse, considering that gladness is presumably a more expanded mental state, generally speaking, than remorse? I remain unsure about such fine points, but I accept that, as ever and always, karma and ethics are fundamentally conditioned and determined by volition—conscious, subconscious, and superconscious. Regardless of whether we consider killing to be wrong, the karmical/ethical quality of an act depends predominantly on the intensity of the volition involved; but that volition is itself conditioned by the aforementioned connection of spirit.
A murder victim, again speaking from the perspective of Buddhist philosophy, is killed according to his/her own karma, which is to say his/her own volitions, since karma is volition; and I can't help but feel that the feelings of the murder victim influence the act and its outcome for the murderer, even if invisibly. For example, we couldn't kill anyone unless it were according to their own volition (this is not to excuse murderers: the killing is the murderer's own karmic act, just as being killed is the victim's karmic act), so their own volitions obviously would have an effect on other's actions—and the other's volitions—toward them. So karma would be contagious, in a sense.
But they say trying to figure out karma may lead to insanity, so I'll stop here. Hopefully it's not too late. And I still slap mosquitoes sometimes, including one that was trying to bite me while I was writing this.