My best friend is angry at me because I don't talk to her anymore?

I don't think you're wrong and you have no reason to feel guilty. She's not being a loyal friend and you trusted her. Her boyfriend is making the situation worse, though. And if she is causing you further problems, I would just not be friends with her- you (probably) have better friends than her and don't need her. I would tell her that you're not comfortable with the fact that shes with him because you're not comfortable with how he treats you and that its not okay with you what he does. Say that you still want to be friends but explain to her how you feel.