Coworker is being very rude to many other coworkers. What to do?

This woman is on some kind of powertrip. I overheard a conversation between her and our new receptionist and she was a total b****. The company has a lot of foreign workers and I am scared she is doing the same thing to them and they're just taking it because they need a job. After overhearing the conversation, I talked to the person it involved and she grudgingly told me some more stuff. Previous employees also told me bits and pieces about this woman so now I am thinking it's a pattern with her and something should be said. She is causing fear in people! The new lady doesn't want me to say anything to our manager even though I said I would be discrete and just say I overheard it, not that she told me anything. I have had interactions with this woman too and let me just say, she has some MAJOR issues. She is not a manager, she is just one of the workers so she has no authority over any of these people she's powertripping over. What to do?