"Why yes, man is completely capable of determining their own meaning to life and values. Total depravity? Original sin? Those sound like spooks, you slave moralist! Anyway, I'm off to do a line of coke and shoot up estradiol mixed with heroin."

There was this previously uncontacted (Amazonian?) tribe where murder was very much part of the male dominance hierarchy. Practically every guy with offspring murdered some rival(s).

This may have been a special circumstance though, because I doubt they had much male cooperation. In more demanding climates, where cooperation is essential for survival this "jungle" model wouldn't work.