I wouldn’t suggest explaining why you don’t like someone. Especially in a social setting…can’t trust people…word can spread…and create for gossip/drama.
I haven’t experience that in the social gatherings I’ve attended…not among my friends.
Hmm, the next time someone badmouths a person…you can try to diffuse/steer the conversation in a more positive direction by either saying a more neutral comment such as “Oh, we’re all different…compatibility varies.”…or…if you know a positive quality of the person that’s being criticized…your can bring that up. So, the listeners have something good to hear about one that’s being gossiped about. Get some reward for doing something you don’t have to do. 