The key, as it has always been through the centuries, is to not think of whoever you're killing as "people". Yes, they slaughtered thousands of people like pig

The key, as it has always been through the centuries, is to not think of whoever you're killing as "people". Yes, they slaughtered thousands of people like pigs, because they didn't view them very differently from pigs. Turns out, in addition to our evolved horror of killing someone like ourselves we also have an evolved ability to dehumanize the other to the point where the "someone like ourselves" label no longer applies. Came in pretty handy, evolutionarily speaking, when two packs of monkeys ended up in close proximity, with insufficient food.