an absolute dereliction of his responsibilities as the commander of that aircraft.
It shouldn't have got as far as getting law enforcement on board, but I'm again stuggling with the urge some have to string the operating captain up on the evidence so far. He/she may have not handled negotiations well, we don't know, but after that?
As an example I have no idea of UA policies but FWIW elsewhere this could ultimately have been construed as passenger "sit in", in which case following legal request from the captain to the individual to leave if an individual still won't move the local law enforcement agencies have to be involved...and, FWIW there has been at least one instance I know of (this time in the States) of an operating captain being threatened with arrest, when he, having called the authorities to resolve an issue he then attempted further intervention. It's not just a States thing, you'll also be politely but very firmly told to stand aside by police if you need their assistance on the ground in the UK.
Fundamentally on the ground it isn't "Master and Commander" of all we see anymore, if it ever was, so I'd cut the operating captain some slack until we have the full story.