And therein lies the really tough part about polyamory (as viewed by some)....
letting your partner do anything they want to do as long as they aren't physically hurting you.
I say physically because emotionally hurting you is a nebulous area. What may hurt your feelings may be something that the other partner could validly want. And just because it "hurts you" doesn't mean (to some) that they shouldn't be able to do it.
So what if she wants to be in a destructive relationship. People learn by trying and failing. It sucks. It really sucks when you're trying to stop someone you love from making a mistake, but if you take the radical autonomy version of polyamory, you shouldn't be able to veto any partner your partner chooses. Take that to the extreme that you're experiencing. The guy she wants to be with is practically going "nanny nanny boo boo" in your face with the whole "I'd kill someone if they did what I'm doing to you to me." Even as dickish as that is, should you be able to veto a partner for being so rude? I'm on your side of course. I'm simply saying that from the poly theory side that a lot of people are on, there are almost on valid restraints you can put on your partner, despite the crappy behavior of the metamour. (You'd hope that the person you love would love you enough back to hate when the metamour was crappy to you...but when blinded by NRE or even true love...that might not happen)
So...all you can do is set your own line in the sand, past which the relationship with your partner isn't worth it. You can communicate what hurts your feelings, but don't expect that to force them into changing their behavior.
Me - male, 42, poly, straight, in a serious relationship with Audrey, also casually dating.
Audrey - female, 20, poly, pansexual, also casually dating.