Inside Human Violence and Cruelty Roy F. Baumeister, Aaron Beck
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A serious effort to get inside the mind of the evildoer.
They argue that there is an incorrect folk belief (well illustrated in fiction) about people who do bad things, which is that they are evil, different from good people, have different ideas of right and wrong, are sadistic, …, and have poor impulse control. In fact, it seems that only the last is true, and otherwise most murderers, etc., are similar to you and me.
Common violence, such as the vast majority of murders, is due to threatened high self-esteem. They are strongly critical of the common social worker view that crime is due to low self-esteem. Generally murder is due to an escalating spiral of violence, rather than any premeditated intent.
To get really major evil, such as the holocaust, what you need is idealism in the service of an apparently greater good.
Though genuine sadism does seem to exist, it is rare, and generally only seems to develop after an initial stage of being revulsed by the violence they have committed. During an early part of the holocaust, the Germans used an approach where individual German soldiers shot individual victims at close range. This was very hard on the soldiers, and was abandoned. One of the problems that the soldiers experienced was “shooting past”, where they would be trying to shoot this person at point-blank range, and they would keep missing. Even though they knew it was their duty, the unconsciously couldn't do it. Ultimately the Germans developed the sophisticated killing systems, with a major principle being that the job of killing was divided between many different people who make different small steps, so the sense of responsibility is diffused.
It is reasonably intuitive that perpetrator has a distorted view of his actions, that what seems minor to him is severe to the victim. What is not so obvious, since we generally tend to take the view of the victim, is that the victim also distorts the events. In fact, in some experiments, it seemed that simulated victims and perpetrators distorted equally. The perpetrator tends to maintain that the act was not so severe, it was a long time ago, and he's a different person now, there were reasons, and he was really forced into it. The victim, as well as smearing the perpetrator as purely evil, also sees the thing as terrible, has a perfect memory of everything that ever led up to this (perhaps going back hundreds of years), and the perpetrator's behavior, rather than being variable, contingent and subject to change, is part of a global and persistent pattern of bad behavior.
I was struck by how these same arguments of grievance vs. defense play out in common arguments over accusations of nonviolent wrongdoing.