Earlier quoted context omitted.
No. It's a federal crime to disobey instructions of the flight or the captain. If they ask you to leave the aircraft you're required to leave. If you refuse and need to be physically removed, tough. Not that I'm saying this makes United's behavior acceptable - I have no idea how they managed to load the plane before realizing they needed extra seats - just that the law is, in this case, on their side.
I'm not trying to rationalize anyone's actions here, but if the captain told you to kill another passenger, that is not within his right of removal of choice for individual. i.e. The captain is only allowed to remove choice for an individual when the group is threatened, given they are in charge, by law, of making choices for group (perhaps for the reason of or related to in-flight matters). Edit: changed he to they.
But also just as certainly he can tell you to leave the aircraft for refusing.
There literally is absolutely no legal right for you to stay on that vessel after the captain requests you leave.
Plenty of incidents support this on a practical level - captains remove people rarely but not unheard of for silly and discriminatory reasons. They likely get disciplined after the fact by the company, but in the moment you have zero recourse as a passenger who was asked to leave. Your options are to grab your bag and walk off, or be removed and possibly go to jail. Either way you are pleading your case on the ground.