Never badmouth others. No matter it’s in a professional or social setting, it’s not appropriate to badmouth people. You certainly will not earn any respect by doing this. If you’re unhappy with someone and what they’re doing, talk to them and work things out. Don’t talk behind their back. That’s the kind of behavior that attracts gossip and negativity. Not only does it reflect badly on you as a person. It also hurts the other party whether you realize that or not.
So be honest and transparent when you’re communicating with others. Always act as if the person you’re talking about is right there with you.