Overcoming a deep-seated proscription against killing is not easy. In his book “Ordinary Men,” Christopher R. Browning described how a German police battalion staffed with fathers, businessmen and plumbers struggled as they executed thousands of Jews in Poland. How many among them missed at point-blank range. How they vomited and cried in the forest after massacring mothers and their children. How hard they had to work at becoming killers.

A culture of authority and obedience that supplants individual moral responsibility with loyalty to a larger mission helps loosen the moral inhibition against murder, social psychologists say. So does a routinization of violence, as well as injustice or economic hardship that allows the killer to see himself as the true victim.

But perhaps the most important ingredient is the dehumanization of the victims, said David Livingstone Smith, professor of philosophy at the University of New England and author of “Less Than Human: Why We Demean, Enslave, and Exterminate Others.”

“Thinking about your enemies in subhuman categories is a way of creating a mental distance, of excluding them from the human family,” he said. “It makes murder not just permissive but obligatory. We should kill vermin or predators.”

The Hutus in Rwanda called the Tutsis cockroaches, the Nazis depicted the Jews as rats. Japanese invaders used the contemptuous term “chankuro” to refer to their Chinese victims during the Nanjing massacre. American soldiers fought barbarian “Huns” in World War I and godless “gooks” in Vietnam.

In Northern Ireland, “taig” was a popular slur for Catholics. Where Mr. O’Callaghan grew up in Tralee, County Kerry, they called Protestants “sassanagh,” Gaelic for “foreigner.”