The largest and most notorious collection of such images, combining both official and “autonomous” photographs, was taken by Nazi photographers, soldiers and civilian supporters: in the Jewish ghettos, on the Eastern front, in the occupied countries, even in some of the concentration camps (Auschwitz employed two official SS photographers).

More recently, members of Sierra Leone’s Revolutionary United Front, famed for their mass amputations of their countrymen, photographed themselves as they committed some of their atrocities. So did some of Saddam Hussein’s Baathists, as well as the Scorpions, a feared Serbian paramilitary force. Several years ago, Hizbul Islam, an Islamist militia in Somalia, invited a photographer to document its death-by-stoning of a man accused of adultery; the resulting photographs are as close to unviewable as any I have ever seen. And some of the most searing and unforgettable images from the post-9/11 era — including the beheading of the Wall Street Journal reporter Daniel Pearl and the Abu Ghraib torture photos — surely fall in the category of perpetrator images.

Other sets of perpetrator photographs display what we might call the “cool gaze” of murderous regimes as they go about their business; such photographs are not always overtly violent, though they are always cruel. Stalin’s police photographed condemned political prisoners in Moscow’s infamous Lubyanka prison before their executions; their faces peer out at us in sadness, fear and bewilderment. The recently revealed photographs from Mr. Assad’s secret jails, though far more graphic, fall under this rubric. Here, again, photography becomes a handy appendage in the bureaucratic manufacture of death.

There is a double horror in the act of looking at perpetrator images. First and most obviously, they show repellent things, fearsome things, unbearable things — things that we would like to believe human beings don’t really do to one another. But this revulsion is intensified by the knowledge that these images were made not as protests against viciousness but rather in celebration, or at least documentation, of it. The very existence of such photographs testifies to the fact that such acts can be cause for satisfaction, for pride, for glory — or even for amusement. (It is striking how many smiles appear in the Nazi photographs and those from Abu Ghraib.)

This is why there is such resistance to looking at perpetrator photographs, and why they are often dismissed as “torture porn” — a handy excuse that relieves us of the burden of viewing and thinking about them. They present a sharp challenge to modern concepts of universalism, to the comforting belief that “we” are all essentially the same and that the family of man, while sometimes disputatious, can unite on at least some basic common values. Not so. Perpetrator photographs reveal how terribly different people can be and how terribly easy it is to excise others from the category of the human. As one of Mr. Assad’s supporters asked of the tortured corpses, “Are they innocent political prisoners or are they Al Qaeda?”