Book with multi personality disorder and murder

This is what I remember: The main character is a guy, and there is a crime (I think it started with like a car explosion or bombs or something). The murderer leaves notes/clues or like death notes or threats or smth like that. The main character has a kind of insane mother who has this false perspective of the world and tries to give him false information (she like cut stuff out of newspapers and like told him crazy stuff and she was going insane). There is a best friend (female) who helps the main character escape from the mother through a window and they hang out. In the end, the main character finds the murderer and realizes that he has multi personality and both the best friend and murderer are him. In the end, he shoots his multi personalities "dead" and he gets together with the detective.