Dostoyevsky is at his best when he sets his characters to oppose each other in a cat and mouse game. The interrogation, for example, was actually fun to read.
But when he sets out to moralize and preach, when he’s not being suspicious but rather ‘in control’, he becomes rather dull.
Then again, perhaps that is the entire point. Madness is more fun than the social norms, at least for the madman anyway.