My childhood friend (26F) is recently engaged. She’s asked me (25F) to be in the wedding. Her fiancé (23M) and I emphatically dislike each other. What should I do in this situation?

I’ll try to keep this short. Claire and I were both raised in the same cult like environment and both abused by our parents. We were super close as teenagers but drifted apart some because I was a typical “bad kid†and got out of the environment, whereas she did everything expected of her and stayed a lot longer than I did. Even though we’re not as close anymore, I still care for her a great deal and want the best for her.
Claire started dating John two years ago, and I think he’s been overall good for her. He’s the second serious relationship she’s ever had, and where the first guy cheated on her and bailed because of her family situation, John stuck by her and helped her to see that the rules she was living by were not normal and she deserved better. Because of him she has her driver’s license, is going to college, living on her own and in control of her own finances, which are all things she didn’t think would or could ever happen for her. So whatever else I could say about him, I’m super grateful to him for that. He also seems to really love her from what I can see of their relationship, and she says he treats her well.
That said, I REALLY don’t like the guy. I’ve never met him in person, but we added each other on Facebook pretty quickly after they started dating. I’m a pretty opinionated person and I’m not shy about it, and I will admit that makes it hard for me to get along with people. However, I can tolerate people who disagree with me on most things as long as they’re respectful. John started commenting on my posts a lot and picking weird arguments (for example, told me I shouldn’t call myself a feminist because it makes me seem unreasonable and like I would scream at a man for holding the door for me, also told me I shouldn’t use the word “dude†casually because “that word is sacredâ€) and once he learned I wasn’t going to just tell him he was right to appease him he started to get rude (at one point I responded to one of his angry comments and told him I actually agreed with what he was saying, we were just using different terminology to mean the same thing. His response to that was “I’m not the problem, your lack of basic critical thinking skills is the problem. We’re really in agreement which you would see if you weren’t so stubbornâ€). It came to a head when I posted something he apparently found extremely offensive (honestly can’t remember what it was specifically, but I think it was something about how I don’t like Father’s Day since my father was abusive). He commented on the post to tell me my opinion was disgusting, PMed me to tell me that if he ever saw me in person he would kill me, and then blocked me.
He and Claire recently announced their engagement and she has let me know that she’d like me to be a bridesmaid. I’m happy for her and I don’t want to seem unsupportive, but I’m really uncomfortable supporting John or tbh even being in the same room as him. I would like to refuse but I don’t see a way to do so without causing a rift in a friendship that means a lot to me. What would be the best way for me to handle this?
tl;dr friend’s fiancé is wonderful to and for her but is also a massive ass who threatened me over a Facebook post. I don’t know how to navigate their wedding and being asked to be a bridesmaid.
Edit because people are asking and I forgot to include it: Claire knows about the death threat he sent me. I assume he’s told her something that makes him look better somehow, because she has refused to take sides and chalks it down to us both having strong personalities and me hating men.


Source.