my partner and i are both poly, in non-hierarchal relationships (i have one other partner they have two, and even though we’re not primaries, we’re the more serious relationship) but i Hate one of their parters. he treats me really poorly when we’re all together and i don’t like how he treats them either, but i don’t know how to bring it up without upsetting them cause even though they’re aware of how poorly he treats them and me, they feel like it’s worth it to stay with him and idk what to do.
You can't control your partner's choices or thoughts - but you can control your own behavior. If someone treats you poorly, you can choose not to be around them. Tell your partner "I don't appreciate how Klavfin treats me and makes me feel, so I'm not going to be around him anymore." That's not you setting an ultimatum, or making a demand. You're not saying "stop seeing Klavfin," or "never invite Klavfin to a hangout" - you're just stating what boundaries you're going to hold.
Your partner might feel frustrated by this, since it's difficult to accommodate around partners who don't like each other, but that's their response to choose. You're giving them information: that you don't want to be around this person. They can respond to that information however they choose.
People may accuse you of "starting drama," but that's not your problem. You get to decide how you want to be treated and who you want to spend time around. If that means you decline an invitation to a dinner, or make yourself scarce when he's around, that's fine! Don't try to get people to pick sides, or start a whisper campaign to oust this guy from your poly network - those usually backfire.
It's frustrating and painful to see your partner stay with someone who, from your perspective, is an ass. And there's a time and place to point out specific behaviors you have an issue with, and remind your partner that they don't deserve to be treated that way. But beyond that, remember that your partner is going to make their own choices, and all you can do is make the best calls for yourself.