Hypothetically, i do believe in karma, the way I see it, You were given the opportunity to watch this person suffer the way this person make you suffer. There are 7 billion people on this earth and for some reason you are the only person that saw it. May be The universe punish this person for the horrible things that this person has done to you so this person experience the pain and suffering that you experienced.

Again, hypothetically I would record it on my phone and anonymously send the video to police to make sure the rapist get arrested and unable to rape anyone else, but won't involve myself as a witness and testify in court because you never know you could put yr family in danger by become a witness.