Cassandra Morphy
The Idea Factory - Killer Identity Disorder
The main character has disassociative identity disorder, where their alt is a hired killer. The story is written in a way that the innocent identity is the only narrator, with time jumps in the story whenever the killer becomes dominant. The book starts out with her waking up at the scene of a murder with blood on her hands, never being aware that her alt had been killing people before.