I've been thinking about Chimpanzees- our (joint) closest relatives and all round evil bastards. They are far stronger than humans and when they attack are brutal- with genitals and fingers being pulled out first before they bite your face off. I'd rather be locked in a room with a tiger or a crocodile than a chimp.
They also practice warfare among themselves, in which the aim is to commit genocide of neighbouring chimp populations. They patrol their own territory and kill any rival males who encroach upon it while also making opportunistic raids into rivals' lands to kill any outnumbered males they find there.
If all this sounds familiar it's because it is what humans have done to each other all too frequently- and continue to do today in many tribal societies- which suggests it is a form of behaviour that has deep routes in our shared evolutionary past.
However in most of recorded history, simply wiping out a vanquished opponent is rare*- a lot of killing occurs but the Anglo Saxons didn't wipe out the Celts, the Normans didn't annihilate the Saxons, the fall of the Roman Empire doesn't seem to have involved genocide of the Romans. Instead elite replacement occurs.
All of which is a roundabout way of saying that the tendency of civilised agricultural based societies to form complex and exploitative hierarchies- in which a self serving elite forms to control the surpluses produced and dominate society may not have been such a bad thing after all compared to the alternative.
An elite that exploits the rest of society will have very little interest in wiping out a defeated population in the way an egalitarian hunter gatherer society does- because it makes little difference whose work they are exploiting.
None of which should is an argument for the perpetuation of social elites today of course, this is just speculation about how we came to be where we are as a species.
* Wars between civilised countries are bloodier overall but not on a per capita basis.