A fundamentalist, myopic and narrow view of this precept is the reason many people believe that Buddhists must be vegetarians. Me? I love vegetarians, they're delicious. I like to eat dead cows. Venison is also among my favorite meats.
As I sit here and write I am breathing. Each time I take a breath I am ending the life of untold numbers of microbes as they encounter the lining of my lungs. When I walk to the kitchen to fetch another pot of green tea I squash millions more under my feet.
Carrots live, until you pull them up to put them in your vegan salad. Life and death are inseparable. One cannot live without constantly bringing about death. This is the way things are. We are all food. My body will be food someday, so will yours.
This walk with death that is a part of all life is not the killing referred to in this precept. This hand-in-hand partnership between life and death is part of the way things are. It is beyond concepts of right and wrong. It simply is.
Of course it is wrong to murder someone, but it is also wrong to kill a friendship, or a conversation, or an idea. What this precept is getting to is that once you've decided that anything is objectionable enough that you willfully end it's existence you have stepped off the path.
In the most obvious sense this applies to another human life. If you decide that you have something to gain from willfully ending another's life because you object to something they do, or are, you are deeply lost in delusion. But anytime that you believe that there is anything out there which is not you, and that ending it's existence will somehow benefit you, you have completely lost sight of reality.
When you realize that everything is interconnected, everything is one huge web of the same thing, you realize that there is nothing out there to kill that is not also you, and any killing you do is really killing yourself, because there is no other thing out there, and there's nothing to be gained from killing yourself.
Or, as one of my favorite Jimmy Dale Gilmore songs goes "it makes no sense, committing suicide in self-defense."
So, the point here is not about murder, although murder obviously falls into the category. It is about this idea that there are things that are here that shouldn't exist. Really? You might as well decide that the sun really shouldn't rise in the east.
If someone were to get up before sunrise, facing east, and as the sun came up scream "No! That's not right! Sun! Go back down! Rise over there! Out of the North!" you'd rightfully call the guys in the white jackets who check troubled souls into the padded rooms. The guy screaming at the sun is completely nuts. What else could that mean?
In the context of this precept, it's just as ridiculous as a guy in Arizona deciding he would somehow be better off if he gunned down some politicians. It's just as deluded. Of course, the consequences aren't the same, the guy yelling at the sunrise is just making some noise, that doesn't compare to the suffering of gun violence, but the delusion is identical.
So, the point here isn't really what one should or shouldn't do, it's about orienting one's view to see things the way they really are. Reality is one thing. Any notion that arises that something shouldn't exist is delusion. If it exists it doesn't require a reason to do so, so there similarly can't possibly be a reason for it not to exist.
This doesn't mean that one shouldn't act when acting is the thing to do. If someone could have prevented what happened in Arizona of course that's what they should have done, even if that intervention would have involved the gunman's death. That's not the killing to which this precept refers.
You will be tempted to create an endless number of such hypothetical scenarios in order to craft ways to deny the simple truth of this precept. That's what our minds need to do in order to persist in this notion that a self exists. Just look at that, it's your mind defending this notion that there is something separate that exists there. This is the fundamental problem.
This notion of separateness, a self and an other, is what is at the root of all human suffering.
This precept serves to remind us that this process, this insistence on the existence of this self, is at work. In the absence of a sense of self/other any notion of killing simply would not arise. There's nothing to kill, nothing to do the killing.
This is how things really are. So, when one is following the Way, that is, seeing things as they really are, one does not kill.