And in the Holocaust you had incidences of this, too -- I'm thinking of Jan Gross' book, entitled "Neighbors," about a small village in Poland named Jedwabne where the Catholic half of the village killed the Jewish half simply because they were given permission to do so. You realize how thin this veneer of civilization is that we put up. We say we live as neighbors and in a community, but when something happens structurally that says now you have permission to persecute, to take from, to even kill people that you've lived with for years, the relative ease with which people can do that is incredible.
It's one thing to understand killing, but killing with brutality and killing with zest and killing by taking trophies as American soldiers did with massacres of American Indians, is another thing. Why is that necessary? You'll even notice that in executions throughout World War II, the person's back is always toward the executioner. There really is no logistical reason for that in terms of ease of killing, it's more just a psychological defense of not having to see the victim.
And, according to Waller, genocide and other atrocities are likely to increase as more people compete for fewer resources.