My fiancee left me for a proven predator, what can / should I do?

My fiancee and I were polyamorous (we dated outside of the relationship with all parties knowing and consenting) her boyfriend seemed sound. However after I spend a week in London with my secondary and she spent a week with him, I got home to things being... Weird. Then a week later she vanishes. She comes back two days later and starts packing her things up and I ask her what's going on, this is the first time we've had a communication breakdown. Her explanations were A) incredibly hurtful and B) completely nonsensical. It was almost as if in her mind things in our relationship had happened completely differently to reality. I then find out from multiple disconnected local people that her boyfriend has a worrying pattern of dating girls of about 17 - 25 years old (he's 43, fiancee is 24) and really emotionally screwing them over, plus multiple rumours of sexual assaults, harrasments, threatening behaviour, stalking and sending threatening letters. As well as at least one rape accusation. He also seems to have two distinct personalities (not like DID, or multiple but more like ones an act for his targets and the other is him without the act).

I still love her, and regardless of whether or not she wants to be with me I want her to be safe and from everything I have been told by multiple people, she isn't safe with him at all. I'm not trying to get her back because that's her own decision to make and there's nothing I can do or should do to influence it. I just can't stand the thought of her with someone so genuinely awful. What should I do?
My fiancee left me for a proven predator, what can / should I do?
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