What about Dominique? Dominique, at first, seems like a friendly and shy girl. She is very patient and kind, and appears as "one of the good ones" to Catherine after Natasha's mistreatment of her. However, Dominique is actually best friends with Natasha, and is very violent. She screams at Catherine, insults her, and says she just wants Catherine to "shut up and die".
(Catherine escapes, don't worry!)
I knew someone like this so that concerns me.