Regardless of whether or not United should have asked him to deplane, once he demonstrated that he would refuse to comply with crew member instructions, the captain wouldn’t have much of a choice but to have him taken off the airplane prior to departure.
If you are not willing to comply with crew member instructions, you shouldn’t be flying.
Ridiculous? So is giving stupid people unchecked power to act capriciously.
It appears that the flight was NOT overbooked. The "guests" were booted (and beaten) to accommodate airline employees. Poor planning on United's part will cost them dearly, and a rent-a-jackboot, or three, their job(s). Rightfully so.