Have you gone to the police? In your entire answer you do not say this, and this is the first step in pursuing actual justice. Making the police aware of this serial rapist is the first step. Maybe they will be able to persuade some of the victimized girls into testifying. I would recommend talking to the police before attempting street justice… but if the cops aren’t going to do anything, maybe your way is the most effective way. I can’t recommend it personally.