But there’s proof that early humans were killing and eating each other as well.
Just like some human are still eating others now. Those are rare behaviors that are not generalizable to the rest of the specie.
Plus, those are not due to genetic but simple social constructs. Various things could explain that behavior, from ideology, to religion, to cultural habits.
We did not magically stopped being savages, were were
never complete savages in the first place. Like in any social groups, early humans were not hegemonous, there are sign that some groups killed each others (probably for the ressources) and there are also signs of inter-tribe mating and coeexistence.
Social structures are complexe. We are not "one" thing. That's why the term "human nature" is not really pertinent. In reality, when we look at it, most violence comes from a place of survival rather than pure discrimination.
This is why for those who value science and know the reality of or social evolution, it's not very hard to imagine a world were we can all live happily, but it's a lot harder for people who think that man is a wolf to man by nature.