OP, people can grow and change. I (44m) was such more of an asshole at age 38 than I am today. It took and takes a lot of work ( reading books, choosing to value learning how I'm wrong over being correct, dropping judgmental attitude, etc). I'm a totally differently flawed dude today than I was just a few years ago.

I was worse than your fiance. Truth is he's insecure (like many of us) and operating under less than helpful narratives. So I think there's hope for him. But you're justified in whatever choice you make.