Forgive a killer?

Reading in the book by James Waller, Becoming Evil: How Ordinary People Commit Genocide and Mass Killing, one idea stood out to me. He says in the book that many people don’t want to study the “evil” of genocide and the ordinary people that commit it because by understanding, you run the risk of forgiving those so-called “evil” people.
The reason it stood out to me was because, from an early age, we are taught to forgive. Whether it’s a kid at school who called you a bad name or your sibling who wouldn’t let you play with a toy, if it became a big deal there was always some mediation of “Okay, now say you’re sorry,” and in response to the apology you’d forgive and at least be civil towards one another for a short time. Now I realize the enormous difference between not sharing a toy and brutally killing people, but “keeping the peace” is a natural human response, it provides cohesion within the group. As Waller points out, how else could all the soldiers fighting for Nazi Germany be able to integrate back into society? That same need for cohesion also brings out another necessary and mature quality, empathy. But how are you supposed to empathize with a person who could kill so easily?
The study that Waller brings up as an example to how people are taught to empathize is Arthur G. Miller’s experiment. It pointed out that people were more apt to understand, or at least be less judgmental, towards perpetrators of an offense after the offense was explained to them. Waller also points out that we explain things in order to make our actions justifiable, but in the end everyone is responsible of their own actions.
Whether people decide to forgive or not, there is still reason to hear the other side out. I think that understanding why people could do so much harm, maybe even how we as a society could just accept those people again, no questions asked, is something that deserves further thought.