They are not mutually exclusive. One can be insane AND evil at the same time. In my time I've known a few insane people who were otherwise harmless, they simply had a loose grip on reality. Once upon a time there used to be institutions for "The Criminally Insane." I don't know if they exist still, but at some time in the past society recognized the difference between "harmless insane" and "criminally insane." The former usually lived out their lives in homes, sometimes with their families The latter were put in soft prison settings so as to restrain their impulses.