I brought in religion as a metaphore type thing, but I've got to disagree. In lots of places one's religion flows deeper than nationalism and if another's nation is trying to wipe out your beliefs then that's reason to kill I'm sure.
I agree with Azrael. I don't, however, believe that nobody is meant to be killed. We're animals and lots of animals kill their own of other packs or as a means to become dominant. Somebody messes up, sure kill em. Only laws that regulate the ultimate right, life, is survival and the laws of nature.
Killing for any reason other than money is stupid though. I say this without being associated in any way with a murder whatsoever.