A good DM doesn't force a player to do things just to appease their own sense of what's proper. It's not the DM's job to police if the player is playing their character right. Their role is to make sure the characters and the story are meshing and interacting properly. In the situation you gave, a good DM would talk to their player and figure out what about this idea of playing a maniac apps to them and work together to figure out how everyone involved can have the best time possible. Maybe that ends up meaning that the player decides to play something different because they agree that the maniac idea doesn't work, maybe it it doesn't. Sometimes a good time in a campaign means going through some serious emotional ringers and intensity, but there's a difference between that and being forced into situations you say at the start you don't want to be in.
I avoid horror books and books with heavy gore because I don't enjoy that sort of thing, but I still enjoy reading stories involving intense emotional hardship. Just because someone doesn't want to experience a certain kind of thing in a game doesn't mean they want no hardship or difficulty. Maybe they want to have to deal with conflicts of people they wronged in the past, or they want to play a character who eventually gets justifiably murdered in the end, or whatever. It's not for the DM to decide when they're playing their character "wrong" unless the way they play their character I'd disrupting the story or the experience at the table.
This exactly. As a good DM you talk to the player and find out, what they really want to do and then discuss together, how to best achieve it.
( I love horror btw, still have to be in the mood to play a serial killer)
