If the mobile phone video represents what actually happened, then UA should be ashamed of themselves. I for one would have encouraged all the PAX around me to exit the aircraft at the same time, then take legal action against United as if that is their corporate ethos I would be concerned for my own safety.
If it is really true, that the flight was only overbooked after 4 UA staff need / wanted to travel then that is doubly reprehensible.
HOWEVER perhaps we should wait for the full story to come out, as often social media's version of the facts can be wildly at variance with the facts. Think back a couple of weeks to the leggins story.