Apt Pupil; Stephen King

A teenage boy, who wants to be a detective in the 1970's finds a Nazi living in his town. He threatens to turn the old man into the cops unless he tells him all of the autrocities committed. Soon the boy is having nightmares himself that can only be released by inflicting pain and killing. The evil grows in him until he eventually snaps when the cops start investigating him after the old man's death.