There's a difference between empathising with struggling friends in pain and internalising their warfare. If Group A criticises me for spending time with Group B, and I then take that criticism to heart, then it's destructive to me.
My responsibility is to do one of several things: (a) don't take it to heart and let them know I don't or (b) take myself out of the firing line, out of an entirely proper self-respect.

Spending an evening, or three, with yourself in preference to spending it with warring friends is not the same as rejecting them forever. It is sending them a message that their behaviour is hurtful to you and that you don't care to have that pain continued.

People who in some way expect you to choose between your friends (if you hang out with A then I'm gonna make you suffer for it the next time I see you) are playing head games with your insecurities about respecting your own choices, whether they recognise that or not. *Their* hurt is *their* responsibility, not yours, and saying that if you don't hurt with their hurt, if you don't internalise their pain, then you aren't their friend, is simply childish.