The ethical question of whether or not to "kill animals" is just one element of a very complex equation. And sorry: I don't believe there is any evidence that evolutionary factors favors vegetarianism.
[WS:] I recall attending a multi-disciplinary conference at UC Berkeley in the mid 1980s devoted to that exact topic. It featured specialists from different disciplines, nutritionists, physicians and anthropologists. AFAIR, the general consensus was that evolution does not favor vegetarianism at all, and we do need meet to survive as a species. There were, however, some important caveats, especially that farm produced meat drastically differs from the game meat that our ancestors ate. Not only in respect to quantity, but also in respect to the quality. Farm produced meat tends to have much higher fat content and the smorgasbord of chemicals fed to animals by the industry.
As to the "ethical" questions about killing - I think it is all rubbish. It is either a religious belief, which is rubbish by definition, or an attempt to hide one's fear of blood and gore under the guise of morality.
I am definitely in the latter category. I am rather thin-skinned and wussy. I would not kill a mouse (my cats do that for me,) but I also understand that killing is as natural as birth and death, so there is nothing "unethical" about it (except pleasure killing, which is perversion in my book.) That pertains to both animals and humans, as the latter are not that much different from the former. But at least I am quite open about my feelings, instead of hiding them under the guise of religion, morality, or ideology.
Wojtek