I’ll keep this short.
If you have a good reason to believe someone is a rapist, your duty is to warn potential victims. It’s very bad when people are falsely accused of rape but incalculably worse when people are actually raped. There is simply no contest.
So be as sure of your facts as you possibly can but don’t be scared of naming names because of what might happen to you.
Another thing: accusations are real things. Repeating them isn’t breaking some kind of code of fucking silence. It has to be OK to repeat allegations. Let’s be aware of the consequences of doing so, yes. But let’s also be aware of the consequences of not doing it.