Recently, I was listening to a "Great Courses" cd on the history of our understanding of evil. There was a discussion of the repeated experience that people who had participated in doing evil things were often convinced that they were doing their duty and what they should be doing. They were not acting like they were demoniacally possessed and did not seem to think that what they were doing was evil. How can this be? I then thought about the fact that often the people doing evil things had experienced mistreatment earlier in their lives. A possible example is the way that Germany and the German people were treated after World War I as they were made to suffer economically and in other ways. The stress of this [abusive?] treatment may have caused many of the German people to cope by dissociating or numbing themselves from feelings and emotional reactions to other people. They would then have been more easily manipulated by someone like Hitler to see other countries as against Germany and believe that protecting their country was their duty.
If this dissociating and numbing happens to a lot of people then there is a risk that they can be manipulated into doing evil things. So, even the people doing evil things may be victims.
This points out the terrible impact that chronic stress can have.