Katrin Bennhold writes: From a comfortable couch in his London living room, Sean O’Callaghan had been watching the shaky televised images of terrified people running from militants in an upscale mall in Kenya. Some of those inside had been asked their religion. Muslims were spared, non-Muslims executed.

“God, this is one tough lot of jihadis,” said a friend, a fellow Irishman, shaking his head.

“But we used to do the same thing,” Mr. O’Callaghan replied.

There was the 1976 Kingsmill massacre. Catholic gunmen stopped a van with 12 workmen in County Armagh, Northern Ireland, freed the one Catholic among them and lined up the 11 Protestants and shot them one by one.

Mr. O’Callaghan, a former paramilitary with the Irish Republican Army, has particular insight into such coldblooded killing.

On a sunny August day in 1974, he walked into a bar in Omagh, Northern Ireland, drew a short-barreled pistol and shot a man bent over the racing pages at the end of the counter, a man he had been told was a notorious traitor to the Irish Catholic cause.

Historical parallels are inevitably flawed. But a recent flurry of horrific bloodletting — the attack in Nairobi that left 60 dead, the execution by Syrian jihadis of bound and blindfolded prisoners, an Egyptian soldier peering through his rifle sight and firing on the teenage daughter of a Muslim Brotherhood leader — raises a question as old as Cain and Abel: Do we all have it in us?

Many experts think we do. For Mr. O’Callaghan, it was a matter of focus.

“What you’re seeing in that moment,” he said in an interview last week, “is not a human being.”

It is dangerous to assume that it takes a monster to commit a monstrosity, said Herbert Kelman, professor emeritus of social ethics at Harvard. [Continue reading…]