A better ending would have been ...

... if in the very last shot the parents would have left the police station with Benny, implying that the police did not believe Benny's crazy murder-story and the parents were called in just to take their son away. In that way, the movie would have given us three shockers. First shocker: Benny cold heartedly kills the girl. Second shocker: his parents cover up for Benny's murder. Third shocker: justice system lets them all walk away. Benny's confession is taken by the police as yet another "confession" by a violence-obsessed teenager who has watched too many movies. And anyway, nobody has reported any missing girl.