SPOILERS for None of this is True by Lisa Jewell

Josie was batshit crazy. Walter had been her mother's boyfriend and she seduced him. The author leads you to believe that Josie killed her husband and tried to kill her daughter, Erin. But really, Walter had a heart attack when Josie threatened to tell the police that Roxie killed her girlfriend, Brooke. Erin attacked her mother.  Josie kidnaps and kills Nathan because she thinks Alex would be better off without a drunk husband. 

Josie is never caught.