Huh? Rome had career soldiers. Don't quote me on the numbers, but a Roman soldier might see 2-5 major battles and dozens of dustups and minor skirmishes. They served for 25 years. Overall death rate was 25-40% but most of that was due to disease or accidents - due to spending 25 years walking around in the woods - not combat. And European medieval wars were fought by part-time soldiers whose main job was making money for their lord. If "few" people survived ancient military campaigns, why would medieval lords fight wars at all? Surely fighting a war isn't worthwhile if at the end of it you have merely a "few" employees left.

> natural act

Most actions do or don't suit circumstances, rather than this being something inherent.

I might take lots of pictures outside at a festival or go pee inside a bathroom, but probably not the reverse.

(See also, the fundamental attribution error.)

What is an unnatural act?

Perhaps you mean only for predators?