I'm not sure if there is anything more infuriating than someone who does something clearly wrong or disrespectful (like seat stealing etc) and then giving you an attitude about it.

I am immune to sob stories and the like that I am sure people use all the time. I've worked the ticket counter/gate and heard it all, apparently everyone needs a free flight move up b/c someone in there family is sick in the hospital, every single person. Desensitizes you a bit I guess in these situations. Just have to be firm and stand your ground.

Only once was I involved in a seat altercation and it was simply me standing up for another pax. This guy decided he was going to take a lady's seat, and she was clearly a flight, let it go and cower away kind of person. He was rude and told her he wasn't moving. So not to get on my soap box, but I told him I was an employee of the airline and I would have no issue getting off the plane and getting security involved if he wouldn't move... he was that abrasive. I probably had zero pull in that situation as I was just a commuting passenger and it wasn't my home airport, but apparently it worked because he moved back, of course muttering under his breath like a tough guy. The FA didn't help at all for her, a little irritating as well.