Should we speak about someone (when he is bad, and I'm sure about that) to aware others?

I met an celebrity lately, and found that he is a bad person, adulterer. Then someone asked me about him today, and I told him the truth. And also made aware some other girls about his bad intentions. A girl became aware about him after hearing it. But it made me thoughtful, whether should I told on his back about his bad characteristic, when it may help other. Or I shouldn't? What does Islam says regarding this. If it is not accepted in Islam then I'll repent to Allah, and try my best to stop this. So, should we talk about someone on his/her back to warn other if we are sure about his/her evil deeds?