Not true that the insane can’t function in society. There are different types of insanity. I give you Ted Bundy. And Hitler.
Don’t forget the jug-earred Marxist
Bundy wasn’t insane. He was evil. Hitler too.
What makes you think that Hitler was crazy? Hitler was a brilliant but evil politician who surrounded himself with evil men. He succeeded in his quest for power for nearly a decade, but fortunately for us he misunderstood and underestimated the nature of his enemies.