It's not his wanting to be out that destroys him, but his being unable to get out of it, precisely. 'time has changed', and Micheal could not again get along as his father once did. No matter whether he chose to purify himself or not, he still wore that mafia face, for he had made his bones and gone onto the way of no return. So it's not that he is a bad Don, it's that the Corleone family is due to fall down.
