Any passenger on that plane had the power to stop that madness at any time by simply volunteering to exit the plane in place of the old guy. No, those passengers just sat snugly in their seats and watched the show; what was obviously a mentally distributed old man in a weird situation.
Fault is everywhere, responsibility is isolated. The law says the captain has the last say, and the total responsibility, even over the police. United is being punished for their repeated past behavior, not for this instance.