One other thing is that he had great talent and competence as a chemist, which was often on full display. Many of the other characters in the show had no clear talents, or if they did, they it was more background or reputation than something clearly shown.
I think any character that shows extreme talent will attract fans, regardless how evil or unhappy they are, because people will divorce the talent from the other attributes and admire it on its own.
I want to be Don Draper because he is handsome, well dressed, and extremely intelligent. That doesn't mean I want to do everything negative he did. Although I wouldn't mind being able to drink at work.
When kids play Batman, one of them has to be The Joker. It could be that they like clowns or the color purple. It doesn't have to mean that the child is murdering cats in the backyard.