Cabin crew can please leave resentments of all kinds at the door and practice some upward management!
It sounds like you and others are now assuming that resentments were brought on board. Possible, but no more valid than other assumptions being made.
I will admit that I find it difficult to get my head around the scenario and how it has been allowed to escalate to the point of diversion (but again I don't have all the facts). I have flown with only two senior cc who I have needed to speak to, one I took to one side, out of site of the crew to have a quiet word...
The above Captain and other airline Captains all over the world (including some who have contributed to this discussion), have proved themselves level headed enough and smart enough to deal with staff issues every day without diverting to an airport unserviced by their company.Whatever the history, if this Captain couldn't deal with the stress of a staff issue without taking the extreme measure that he did, his ability to deal calmly with other on the job stresses could legitimately be called into question. That's not the same as saying he is 100% guilty of anything
Also, if United Airlines had any doubt about the purser's behavior in this, wouldn't she be on suspension until further investigation? The fact that they didn't suspend her says a lot.