This has happened many times, and it will happen again. After each of these crimes, we lose our innocence—but then innocence magically returns. In the aftermath of horror, grief, and disbelief, we seem to learn nothing of value. Indeed, many of us remain committed to denying the one thing of value that is there to be learned.

After the Boston Marathon bombing, a journalist asked me, “Why is it always angry young men who do these terrible things?” She then sought to connect the behavior of the Tsarnaev brothers with that of Jared Loughner, James Holmes, and Adam Lanza. Like many people, she believed that similar actions must have similar causes.

But there are many sources of human evil. And if we want to protect ourselves and our societies, we must understand this. To that end we should differentiate at least four types of violent actor.