These photos teach us an important historical lesson: that there is no line separating humans from monsters, a single being can be both, simultaneously.

Contrary to the common presumption, you do not lose your humanity simply because you dehumanize others. Even our worst enemies and the most hideous criminals are startlingly normal people.

If there is a difference between the good and the bad, it must be that truly good people do not think themselves above being capable of true evil. When we lose sight of that, when we begin to presume our own righteousness, declare those who differ with us irredeemably mad or evil, and excuse our own transgressions on account of our own faultlessness, who is to say that we are any different from these people?

Museum curators have avoided describing the album as something like "monsters at play" or "killers at their leisure." Cohen said the photos were instructive in that they showed the murderers were, in some sense, people who also behaved as ordinary human beings. "In their self-image, they were good men, good comrades, even civilized," she said.

In our self image we are also good men, good comrades, and even civilized. But what crimes or ours have we excused on that basis?