People with mental illness are incapable of rational thought and appropriate action -- they unwittingly *cannot* think rationally or act appropriately -- where "evil" people wittingly *will not* employ rational thought to arrive at appropriate attitudes and actions. Hence, anyone with a self-serving, ego- or pride-driven motive -- an act of one's own will -- for committing an evil act is not "mentally ill" (and possibly still not inherently evil or "criminally insane"). For example:

I once had myself locked up (voluntary commitment) for a while because I believed a psychotic break or whatever was looming on the horizon and I did not want to commit homicide. In contrast, there have been times when I have *willingly* considered homicide for some kind of personal satisfaction. A court would have considered temporary insanity as a plea in the first case, but the second case could have stemmed from nothing other than conscious criminal intent.