I'd say that Humans are capapble of killing because we evolved as a preditory species. We do not produce our own food from the sun, from heat, or anything else. we are incapable of eating rocks or dirt and living for very long, so we have the ability to kill. Had any individual during our evolution refused to kill anything in the name of their own survival, thatindividual would not have survived long to produce kids.
So we as a species kill be cause we have to to live. our food comes from the imposed suffering of other beings.
As an extention to that, to keep our reproductive possibilities open, we need an ability to fight of other individuals encroaching on our territory, so we need an ability to mentally and emotionally survive killing one of our own. In tandum to that, for the sake of species survival, we have also evolved a sense of guilt in killing one of our own through empathy. So for the msot part, we don't go randomly killing each other; we reserve that for special occations.
What would make me kill? If I were in a position to stop the death/suffering of many people through the death of a much smaller number of beings, and the long-term effects (7th generation methodology) were not overly negative, then I would very possibly kill.
Having never killed anyone myself, I can't say for sure, though. I'd like to say that if someone was attacking me, and there was no benifit in my surviving over the other person, I would let them kill me. I doubt it, though, as I tend to react in self defence when attacked. so all in all, I can't say until it happens. Then I'll come back and post my expirience.
Tool put things into a good perspective for me with the song "disgustipated":
http://www.lyricsfreak.com/t/tool/3155.htm