The problem here is the word sanity. Nobody thinks John is insane in the sense of being disconnected from reality. The doctors just think John is dangerous - a psycopath. They're using the word "insane" in the sense that it means "someone who needs to be locked up". John is either asserting that their invocation of that sense is inappropriate for somebody with his neural condition, or he is lying about himself, (which I'd say is more likely). Psycopaths have no great trouble diagnosing themselves. They just don't go around telling other people.

John is right though. He's not insane. He's just evil. Whether we ought to treat that in the same places we treat the insane is a good question.