Fundamentally, civilization is about control. The control is codified and accepted only because there is a certain amount of inhumanity that is required for it. I would argue that killing comes naturally to humans, and other acts of similar cruelty.

Morality and ethics are learned behaviors that are distilled from societal mores. If "thou shall not kill" or some equivalent in another religion didn't exist, for example, I'd argue killing would be natural in society, and nobody would question it. That is why certain tribal and indigenous peoples performed human sacrifices and had various rituals involving bloodsport. To those tribesmen, killing is not something abnormal within the tribe. Why would killing someone from another tribe matter to them?