Call me crazy, but I don't think serial killers can get away with their crimes scot-free just by pointing to unmurdered victims and saying "They did it!" Bill would be considered a suspect along with Jennifer and Craig until the police could start an actual investigation, at which point the abundant evidence of his guilt would be made glaringly obvious. As a sudden shocker ending it sort of works, but you kinda have to not put any thought into it.
