Need help understanding the dad/child/ex wife dynamic

I hope this is ok to ask here, I’m trying to get perspective. My fiancé and I just broke up, he was divorced for 12 years now and has 2 teen daughters. His ex wife has had numerous live in relationships, and a second marriage/divorce during that time. I am the first relationship longer than a few months for him, the first person he lived with and proposed to since then. His ex wife ended her last relationship when I moved in with him and essentially blew up her life, and for two years from that point on she made our life miserable. His daughters liked me, his family loved me, and the closer we got the angrier she seemed to get. It seems she turned the kids against him, the barley talked to him this last year, went from 50/50 custody to them with her full time and not wanting anything to do with them. 6 months ago there was an argument at his door with her, the kids were there (17 & 14), they were arguing about me and I don’t even understand why. She kept saying he was trying to force a relationship ship, he was saying all the times she was with someone he supported her. I tried to get her to stop yelling, asked her to just come in and talk. Next thing the 17 year old punched me in the head and slammed me into the door jam. I was shocked, he did nothing, and I said if she ever puts her hands on me again I’ll knock her out. It was crazy. They left, he did nothing about her hitting me, and for 6 months they didn’t speak to him and said if he was going to marry me then they were out of his life. He tried to reconcile with them, they wanted no part. I apologized to the ex and the kids several times for interfering and asked if we could just talk. Nothing. Finally, 3 weeks ago, he broke down and said “she wins, she beat me, I can’t have my kids and you, she beat me.” And we ended, I moved out. I can’t understand any of it, I can’t understand her having a say over his life, I can’t understand him giving up. As divorced dads, would you give me your take on this and what he was experiencing? Is any of this normal? Is there a chance he regrets his decision? Was it the right decision, as a dad would you have done the same? I’m just looking for insight or perspective, any you can offer would be appreciated. Thank you.