Mr. Brooks (2007)

Well-respected businessman Earl Brooks (Kevin Costner) has a secret: Beneath his family-man facade lurks a murderous alter-ego named Marshall (William Hurt). Brooks struggles with his addiction to killing, but Marshall enjoys his grisly hobby too much to let go. Finally, Brooks resolves to commit one final murder, but a voyeur (Dane Cook) witnesses it and blackmails him to continue. With a detective (Demi Moore) hot on his trail, Brooks/Marshall must spin an increasingly elaborate web.