My own inclination is to wish that the excuse were more fully psychological. Tell us more. Of course, I ask these questions in jest. The clear purpose of this sort of explanation is not to reveal but to hide. To my mind, it beats political puritanism or McCarthyism.
Normal people are capable of great evil.
I certainly agree that violent, extremist subcultures may attract the mentally ill. It may be that I see a stronger connection between affect and cognition than you do. I find that when they are less depressed or anxious, people tend also to be more flexible in the range of explanations they will entertain. That said, psychotherapy is hardly a cure for racism. For the profession, that means striving for greater clarity in our diagnostic system and greater openness about its limitations.