Revenge is an act designed to inflict harm on someone because they’ve inflicted harm on us. We could yearn for anything after we’ve been mistreated, like a scoop of ice cream, a nap, or a relaxing massage. But what most of us really want is the other person’s pain—and for them to know that their pain is because of the pain they’ve caused us. The desire for revenge is the root motivation for almost all forms of human violence. From childhood bullying to intimate partner violence, urban violence, police brutality, mass shootings, violent extremism, genocide, and even war, perpetrators of...