As Hannah Arendt foresaw, we are once again up against the question of evil. An American photojournalist, James Foley, was presented to the camera and methodically decapitated. The instrument was not the ax reserved for royalty or the whooshing blade prompted by that reformer Joseph-Ignace Guillotin, but an ordinary looking knife. Death would be neither swift nor painless. This, somewhere in the bleached desert, was pure evil.

I used to not believe in evil. When Ronald Reagan called the Soviet Union “the evil empire,” I thought it was a dandy phrase but also a confession of ignorance. The word itself connotes something or someone diabolical — bad for the sake of bad. The Soviet Union was bad, I conceded, but not for no reason. It was bad because it was insecure, occupying the flat, inviting, Eurasian plain, and because it had a different system of government that it dearly wanted to protect. Reagan had it right, though. The Soviet Union was evil.

Now we are facing a different type of evil. The Islamic State, in whose name Foley was beheaded, murders with abandon. It seems to love death the way the fascists once did. It is Sunni, so it massacres Shiites. It is radical Sunni, so it eliminates apostates. It is Muslim, so it kills Yazidis, a minority with a religion of its own, and takes, as plunder, their women as concubines. Men are laid in a ditch and shot in graves of their own making.

The Nazis are back — differently dressed, speaking a different language and murdering ostensibly for different reasons, but actually for the same — intolerance, hatred, excitement and just because they can. The Islamic State’s behavior is beyond explication, not reacting as some suggest to the war in Iraq — although in time it will try to settle some scores with America — but murdering and torturing and enslaving because this is what it wants to do. It is both futile and tasteless to lay off blame on others — the West, the colonialists of old or the persistent Zionists — or to somehow find guilt in the actions of the rich or powerful because they are rich or powerful. You can blame the victim. You can even kill him.

In the weekend Financial Times newspaper, the British writer Martin Amis tackled the question that obsessed Arendt and so many others -- the nature of evil and its ultimate personification, Hitler. Amis mentions some historians who have attempted to understand Hitler — none of them succeeded — and settles finally on Primo Levi, the great Italian writer of the Holocaust who was sent to Auschwitz. It was there, when a guard “brutally snatched” away an icicle Levi had broken off to slake his thirst, that he asked in his poor German, “Warum?” The guard replied, “Hier ist kein warum” — there is no why here. There was no why in all of Auschwitz.

Amis, too, has looked for the why. His latest novel, “The Zone of Interest,” is about the Holocaust and he too cannot find a core of reason or explanation in the events that change our understanding of who we are. All fall short. It’s as if they bump up against the camp’s electrified fence and, with a hideous sound, are evaporated. Amis leads us to Levi’s book “The Truce” and a passage he had not seen before. Levi had made his peace with not understanding. “Perhaps one cannot,” Levi wrote, “what is more one must not, understand what happened, because to understand is almost to justify.”

You see this happening now. The atrocities of the Islamic State are attributed to the Iraq War and what it did to the region. In the current edition of Foreign Affairs, John J. Mearsheimer, a University of Chicago political scientist, attributes Vladimir Putin’s bad behavior to the West’s bad strategy. “Why the Ukraine Crisis Is the West’s Fault,” it is titled. Putin is almost entirely effect. Everyone else is cause.

Putin is certainly no Hitler (or Stalin). But the category of evil remains useful. It assigns agency where it belongs — not with the victims or even with the enemy, as Western leftists did for too long with Stalin, but with the person or force that is doing evil things. The decapitation of Foley and the depredations of the Islamic State are evil returned, evil that can be understood only as beyond understanding. It needs to be eliminated. More than Foley was killed that day in the desert. So was the why.

Richard Cohen is a writer with the Washington Post Writers Group. He can be reached at cohenr@washpost.com.