IMHO, most anthropologist would agree that the human species has evolved so far from any thing close to nature or natural that only the social anthropologist would study us. (snip)
I disagree. We have not evolved much at all in the past 6,000 years -- a mere blip on the timeline -- and possess virtually the same genes (and potentials) now as we did then.
In the few remaining isolated (human) hunter-gatherer populations, you'll still see human natural instincts in their original, "natural" milieu; they are still our own potentials. If we were to look at their gene pool next to our own, they'd be virtually identical.
Over the past 60-80 years, there have been studies on the world's remaining hunter-gatherer populations. A number of them were known to regularly raid each others' villages and kill their rivals as a natural part of their lives. Theories came down to natural population control, access to (perceived) superior resources (including women), natural selection (obtaining mates from a source outside one's village = more genetic variation/health), etc.
In other words, this is a natural form of aggression that may actually maintain a healthy population- and genetic balance in a human population. It is echoed by similar behavior in primates such as langurs and chimpanzees, who also are aggressive and have been known to kill members of their own species.
I believe that the ability to kill is one of many long-extant survival mechanisms, its natural triggers being 1. self-defense and 2. stress created by population pressure (both actual, physical stress and the stress of limited resources such as water, food, space and access to mates).
There is a tendency for us (post-industrial societies) to see ourselves as representing all humanity -- urbanized, "self-domesticated" -- believing that we alone constitute Modern Man. Perhaps that someday will be true after we have forced the remaining "People of Nature" to move to cities and wear shoes. Yet, the same reflexes and instincts are intact in us as in the peoples of early-20th-century New Guinea and the Amazon; we just express (or repress) them in different ways which reflect our own milieu.
Edit: While I was typing this, David Henderson posted his thoughts. I agree with them, and they mesh with what I have written (making my comments a little redundant!