This week I read in Paul Seabright book.
There is ample evidence that hunter gatherers were extremely violent. Seabright argues a pro-murder gene was highly beneficial (in terms of reproductive capacity) back then, mainly as apes became more intelligent. Here is why: two stupid lions or deers fighting for a female or a territory do little physical damage to each other ( u can see this in discovery channel). The weaker quickly acknowledges his inferior conditions, leaves and DOES NOT RETURN.
Now, a shrewd ape could easily quit the fight only to later sneakly return and impregnate the winner's female. That being the case, killing him would entail a good reproductive benefit.
And hence the murder gene passed on...