From what you've told me, this guy A sounds like he is very manipulative. It's not YOUR fault, it's guy A's fault. He also sounds like he has an anger problem.. if it gets serious enough and he is actually serious about saying he wants to kill guy b.. tell someone! Even a killing threat is WRONG! I wouldn't suggest going out with guy A, he doesn't deserve you with the way he is treating you and your b/f. So maybe just stay out of his way for a while so he can cool off..