My question involves criminal law for the state of:

Lets say that you know that a person is a rapist. 3 years ago you confronted the person about a rape and the person admitted the rape, but said he would kill you if you told anyone.

You decided not to report the rape out of fear. You heard about the rape through word of mouth. You don't know the woman's name, but you know the neighborhood she lived in at the time of the rape.

The woman never reported the rape. Then years later you feel guilty and report the rape to the police, the police say they can't do anything, because no one reported a rape in that neighborhood and also the victim is unknown.

You could send letters to everyone in the neighborhood and also check records and send letters to all the people who moved out, with the name of the rapist.

However; instead of doing that you send a letter to the rapist and give him an ultimatum.

You say "either you get your testicles in penis cut off in a medical procedure or you turn yourself into the police for rape, you have 7 days to do this"

Is that a crime? To say that to the rapist?