[mst]

Murdering all of the men of the losing side of a war and taking their women as slaves to bear more of your children is evolutionarily adaptive - and I would presume that as a result I'm descended from at least one person who did so, and so would be the person I was talking to in your scenario.

"Adaptive" and "morally horrifying" can both be true of the same behaviour.

I hope that the things I consider moral win in the long run, but that's unfortunately not guaranteed.