There's a school of thought which holds that insanity is the very nature of evil.
Scott Peck talked about the "malignant narcissism" of evil in his book People of the Lie," and I believe that was present in the few people I have known whom I considered to be truly evil. These people did not routinely throw fits of anger. That would have diminished their ability to get close enough to others to do real harm. Instead they were intelligent and superficially plausible.