ZD – what you are saying would only apply to someone who is a molestor, not to someone who cheats on his taxes or does other aveiros.
Even regarding someone who is a molestor, there are still many halachos and issues involved and you have to ask a sheilah. The fact that someone has been accused of molesting someone at some point in their life does not necessarily give everyone in the world a blanket heter to talk about it whenever and wherever they feel like it.
For example, there was a boy who went through a difficult period of time in his life and did not act appropriately with girls, although there is no reason to think that there was any “ones” involved. He went to therapy, dealt with his issues and turned around and was totally frum and not a danger to society in any way.
After he got help and had turned around, someone went and posted about him online “warning” people to stay away from him as he was a molester. If you continued reading the email carefully, according to what the person claimed he did, he wasn’t even a molester. Unless you think that girls are always victims and not responsible for their own actions, which seems to be the contemporary approach but is ridiculous. And certainly doesn’t justify speaking LH about someone.
Additionally, he was fine at that point and not a danger to society. So it was really disgusting that someone went and shmeared him publicly like that, especially after he had worked really hard to do teshuva and get back on track.
The point is if you think it’s mutter to say something because you think there’s a toeles: Ask a sheilah. It’s not so hard. There’s a Shmiras Halashon hotline you can call any night between 9:00 – 10:30 (I think). I think you can find the number online.
There are many conditions required for something to be allowed. These conditions are rarely fulfilled.