Caught in the crossfire

I started dating a lady over a year ago and was really enjoying getting to know her and I admit I was excited about the future and then she basically vanished.. we were talking every other day or so, texting several times a day, and hanging out twice a week, when she cut all communication. A month later I get a text, “hi”. She had resumed a relationship I viewed as toxic. She completely supports her boyfriend and she is totally reliant on him emotionally. Craving what he will never give her, his acceptance. Discovered recently that he is also physical abusive.
Not my monkey, not my circus, but I’m constantly dragged into it. After a fight, either verbal or physical I am called to pick her up and validate her until she is no longer the scared little mouse shivering in a corner. (The woman I met was more likened to Katy Perry’s “roar”). At that point, she calls him. “To find answers”. I know what the outcome will but i will never take away her choice.
I pick her up last week after she had soiled herself from being beaten so badly. Take her to my home, nurse her back to a fragment of her former self. I resist my own desires to let her jump into a relationship with me and offer the advice that she needs to take 6 months to be alone with herself, and me as a friend by her side.
Long story short, she is with him again this morning.
And he is a piece of shit, recently released, sex offender, chomo, no job, she pays all the bills, and he constantly cheats, (4 times he has admitted to and suspected another 5/6 times).
She knows what the outcome will again be. I’m feeling guilty because a) I knew exactly where she was going when she left and am doubting letting her b) I told her I was not willing to be used as a pawn in whatever game she was playing with her bf.
C) I have this overwhelming desire to beat him to a bloody pulp and that’s not who I am. D) maybe I should have said yes to the proposed relationship it is what I want, I was attempting to do what was right for her

Within 3 weeks, she will call me. Tell me of the abuse de jour and ask for help.
What should I do?