George Harvey is eventually suspected of being a serial killer, which he is, but he is never caught or punished for the crime. There was never enough definitive proof to arrest him.
At the end of the novel, Susie moves on into a larger part of heaven but she is still able to watch events down on earth. One day she sees Harvey getting off a bus outside of a restaurant in New Hampshire. Harvey sees a young woman near the restaurant and tries to accost her but she refuses to speak to him, lucky for her. Susie notices some large icicles hanging from the roof. After the woman leaves Harvey, one of the icicles falls and hits him on the head, he tumbles down a ravine and is ultimately killed.
So, he is punished by a higher power in the end and the reader is left hoping that he will have some explaining to do wherever he winds up going in the afterlife (this is my addendum, not part of the book).
This novel is similar to the "magical realism" made popular by several Hispanic authors. Magical realism blends magical or spiritual elements into a realistic atmosphere in order to arrive at a deeper understanding of reality. The novel is realistic in most of its elements, but then there is the element of the afterlife, the spiritual, etc., which requires a reader to venture into spiritual realms.
In the movie, Lindsey does. She sneaks into his house and finds a book that he has. In the book he drew pictures of the underground 'cave' and he had a lock of her hair.