Either act, or forget.

Talking about someone else behind their back makes no difference to them, and all the difference to you. Your friends have to put up with you, you get frustrated, and sharing with your friends only reaffirms your complaints and causes them to perpetuate. I.e., in order for you to be consistent with what you say, you will find more and more evidence (that someone is bad/wrong/evil/stupid) to back up your word and “prove” to your friends your point of view is correct. The vicious circle continues with you and your friends, while the complaint (the someone) lives happily in oblivion. They are indifferent, you continue poking and you get to “be right”.

It doesn’t seem fair, does it? They’re the complaint, and you and your friends are the ones that suffer. What’s that about?

Give it up! And clean it up! Tell your friends you’re going to stop talking about X behind their back, tell X you’ve been speaking about them behind their back, but that you’re not committed to that anymore, and just let it go!

Complaining is silly. Either act, or forget.