Ultraviolet - R. J. Anderson

Once upon a time there was a girl who was special. This is not her story. Unless you count the part where I killed her. Sixteen-year-old Alison has been sectioned in a mental institute for teens, having murdered the most perfect and popular girl at school. But the case is a mystery: no body has been found, and Alison's condition is proving difficult to diagnose. Alison herself can't explain what happened: one minute she was fighting with Tori - the next she disintegrated. Into nothing. But that's impossible. Right?