Intergroup aggression started as a meme that is adaptive in cultural group evolution. Then, in turn, it created biological selection pressure in the gene-culture coevolution process. it's more than likely already an evolutionary mismatch.

For most people, does a new understanding of human nature actually change our behaviors? E.g. We came to understand that evolution made us care more about our own children than our neighbors'. Will that understanding change our behavior? What if we come to understand that evolution made us prone to hate people from outside groups? What does history tell us about this?

There were many examples of the impacts Darwinism made on societies. With a secular understanding of where the human species came from, humans became more accepting of human nature with religion fading further to the back seat. For example, sex was no longer deemed a sin, self pursuits of pleasure (without negatively affecting anyone else) are all natural and nothing immoral.

The Christian belief of human souls and the original sins was one of the deciding factors of collective behavior in Christendom until the Enlightenment changed those beliefs with updated understanding.