Individual people have no shortage of selfish motives for violence. But the really big body counts in history pile up when a large number of people carry out a motive that transcends any one of them: an ideology. Like predatory or instrumental violence, ideological violence is a means to an end. But with an ideology, the end is idealistic: a conception of the greater good.
Yet for all that idealism, it’s ideology that drove many of the worst things that people have ever done to each other. They include the Crusades, the European Wars of Religion, the French Revolutionary and Napoleonic Wars, the Russian and Chinese civil wars, the Vietnam War, the Holocaust, and the genocides of Stalin, Mao, and Pol Pot. An ideology can be dangerous for several reasons. The infinite good it promises prevents its true believers from cutting a deal. It allows any number of eggs to be broken to make the utopian omelet. And it renders opponents of the ideology infinitely evil and hence deserving of infinite punishment