That applies to all living creatures in the case of Buddhist teachings. It refers to any intentional killing of any living creatures, even insects - but the complete breaking of the precept only happens if you kill a human.
However, you are not out on a crusade to save all creatures from dying in the ordinary sense - recognize that they all will die anyway. The point is rather that you are training yourself mainly - learning to be come more open and sensitive to others.
That's why a Buddhist monk, or nun, even one who keeps the precepts strictly, can accept and eat meat if offered it by others - so long as it is not killed just for them. So, a Buddhist monk or nun can accept e.g. if you say "Please eat this chicken". But not if you say "I've got this chicken in the yard, please wait a minute and I'll kill it and serve it to you". If you don't understand the reason for the precepts you might wonder - what's the difference, the chicken dies either way? Some Buddhists are vegetarian also, but you don't have to be a vegetarian to be a Buddhist monk or nun or to take the lay vow of not killing.
Where it gets a bit more tricky is - first that these precepts are there to help you to find your way to true wisdom and compassion. They are not by themselves the final thing, the be all and end all of Buddhism.
Also sometimes Buddhists break their precepts for other reasons. Just because you take them doesn't mean you will automatically be able to follow them perfectly. As imperfect humans we make mistakes, or find ourselves in situations where tricky decisions have to be made.
So e.g. one of the precepts is about intoxicants - and some people are especially susceptible to intoxicants, maybe easily persuaded to get drunk. If you take intoxicants to the extent that you get drunk - you may well in worst case then break all the other ones as well in your drunkenness.
Or you might just get angry and in moment of anger break a precept. Or forget about them, get caught up in your situation and forget you took them. Or deliberately break them. Just because you are a Buddhist or have taken some vows doesn't mean you'll never lose your temper or be forgetful.
So a nun or monk being only human could do any of those things.
Other times also, a nun or monk could deliberately break the precepts, not out of deception or forgetfulness, but out of compassion. If that happened then it's a case of handing in his or her robes to cease being a monk or nun - but it's recognized that such situations do arise,
There's a famous example of the Buddha himself in the stories of his previous lives. This was a merchant who was plotting to kill 500 other merchants - and there was no other way to stop him.
The context for this is - the idea that if you do violent actions you end up in the Buddhist hell realms. Now this is not like our Western idea of hell. There is no Satan figure sending you there.
Rather - the idea is that if you do very violent actions - then it changes the way you see the world and relate to it- in a way that makes it really hard to find true happiness because of your changed perception.
The Buddhist hell realms then just refer to beings just like us - but more sensitive to pain and with longer lives, lasting millennia or longer, who can experience extraordinarily high levels of pain - and who experience them for extraordinarily long periods of time.
Though we don't have direct experience of this in our lives - by extrapolation, we know of beings with longer lives, and - can understand it might be possible somewhere in our universe or some other plane of living (they also have the idea of beings who can experience amazing levels of pleasure for long periods of time also - but who eventually die).
Whether you believe that or not - you don't have to, to be a Buddhist, and that's not what this question is about - just to understand the background of this sutra, what most readers of it would have believed at the time. The idea is that someone who kills others is also harming themselves in a way that is hard for them to deal with.
I suppose in present day - would think of things like e.g. the Vietnam veterans who can't sleep, hole up in huts in the middle of forests, and warn their closest ones not to wake them up in case they murder them - all side effects of the violence they engaged in during the Vietnam war - and that's just in this life. If you do accept that something continues to other lives - then one can understand how even worse things could happen to you as a result of extreme violent actions you do in this life, when you are no longer tied down by the limitations of the present body, which provides some stability and limits the levels of pain you can experience to some extent.
But - it's not like there is some ledger book as in popular or cartoon book type ideas of Karma - not like that if e.g. a mosquito bites you on the nose - that you must have bitten it on its nose in a previous life. It's much more kind of fluid and flexible than that, changed perception - but you can also purify yourself, change your perception back again. Someone who has done extreme violence in this life can do a lot to work on the effects in this very life. One of the Buddha's students was a notorious mass murderer of his time, ho did just this, completely changed his life around.
Anyway - whatever - so - but sometimes a Buddhist might still kill someone out of compassion as the Buddha did in his previous life. And (if you think that way) you might do that even knowing the effects such a violent action of killing someone might well have on your self in future lives, but your compassion overrides that, you do it anyway, you can't bear to see all these people killed..
A more modern example - if you saw someone who was about to explode a nuclear weapon in a city - and you could stop them - but only by killing them - no time to try to reason with them or try to get them to change their mind - that would be a clear case where you might well kill them even though it goes against your precepts - even if you are a monk or nun, or even taken the full monk's vows. You'd do it to prevent the suffering of all the people they will kill - and also to prevent the suffering of the person themselves in future lives - if you think that way at least.
And - but would be very hard to do that with total compassion for the person you are about to kill - without any anger and any hint of wanting to hurt them. But we are imperfect humans and have to do imperfect actions sometimes.
One of my teachers said that in his opinion, if you had the opportunity, as a Buddhist, to kill Hitler in the second world war - then that also would be a similar situation - though that sort of thing is more tricky - you have to decide on an individual basis, and are going to make mistakes. When it gets to e.g. Bombing of Dresden in World War II of course it gets far more contentious.
Luckily most of us don't have to make such life and death decisions in our daily life - though you have many other tricky things to deal with of a lesser nature - and that's just the way our lives are..
(It's getting a bit tiresome for the reader, I think, to write "monk or nun" all the time, so will just say monk in what follows - please understand as meaning monk or nun).
So - that's the context - there is some background of possibilities for monks to be violent to the extent of killing even - but if you did that you'd need to give up your monk's robes, you'd no longer be a monk after that.
There's the idea in the West that Buddhists have to be pacificists - they don't have to be. You get Buddhist soldiers in Buddhist countries after all, even historical Tibet had a small army of its own. Is pretty hard to reconcile on the personal level - if you are a soldier and a Buddhist while at the same time following the teachings on developing compassion to all beings without exception - still - are sutra texts about it also, and the Buddha gave special teachings to monarchs of his day about right conduct when defending their country against invaders.
Buddhist monks certainly have to be though, they've taken a vow not to kill, You renounce all such violent activities when you became a monk
When it comes to monks who kill themselves - is same thing, violence against yourself as others - you are killing a living creature both ways so prohibited by the same vow. So - but there is some precedent there also of a monk who committed suicide in this way out of compassion in a tricky situation - but whether all monks who commit suicide are doing it out of compassion in this very unusual situation is another matter.
That's why e.g. if you ask the Dalai Lama to comment on the monks who kill themselves in Tibet - you really can't expect him to say anything - except of course if they were to go to him for personal advice. But it's not for him to tell them what to do - just doesn't work like that. There is no such thing as a Buddhist pope or overall authority on what you should or shouldn't do.
In Buddhist teachings these are all individual decisions - you have to decide for yourself.
However for sure - the movie idea of monks who devote their lives to learning special skills to help them to kill others in a kind of Buddhist monk's assassins guild or some such - that's so far from the truth of Buddhist monks in practise as to be laughable.
Perhaps it's based on the Samurai? But though they were Buddhists, they were not monks, though later many of them gave up their violent ways and became monks Samurai Philosophy
Monks are only human, and could be violent, but if they engage in warlike activities and kill people - would need to give up their monks vows. Or should do anyway if they are within the genuine Buddhist traditions.
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Been following this up a bit more, the Japanese Sōhei (linking to the wikipedia article but see its talk page) did fight and kill it seems - but probably not ordained so not really monks. And the Shaolin Kung Fu - training in martial arts to defend yourself, or as a Zen practise - that's not breaking any vows if you don't use it to kill people. Perhaps it's a mix of those two?
Anyway to fill it out a bit more, here is an introduction to the Buddhist monk's vows - Therevadha tradition I think - the Tibetan Buddhist monks are not so strict in the guidelines.
The Buddha said that some of the vows are minor ones that don't need to be kept strictly - but no-one knows which ones were the minor ones and which the major ones - so though the actual vows are the same, and major breaking of the vows I think would be the same - but how strictly you keep the rest of the vows is a matter of personal judgement and varies depending on the tradition.
Anyway on killing people, it is clear enough. If you kill someone after you take your vows, you have to hand back your monk's vows and can't be a monk again in this lifetime.
On the rest, e..g. Tibetan monks often handle money, eat after midday like everyone else and don't necessarily wear monk's robes either in everyday life - while the Therevadha monks are much more strict about those things.