Brainwashing/cult/fall into insanity.

I read These Violent Delights last year by Micah Nemerever and absolutely loved it.
I’m in the mood for something slightly similar.

I’d love a male main character if possible. Any books about a person slowly falling into madness, becoming obsessive or being lured into a cult. Something dark that explores how someone normal or maybe vulnerable can find themselves in a slightly sinister position.