Well, he was more than an actor. He was a public figure, a standup comedian, a PhD holder who spoke often about educational and cultural issues.
This is more than a case of just someone who plays a good guy turning out to be a bad guy. His fame and influence extended well beyond his character work.
There are lots of people with a public persona of one thing and a personal mind, life and character of something completely different. Sex perverts/predators usually have an upstanding public persona which gets them access to victims with a misplaced trust. Often they have power over the victims to keep them quiet.