Facebook is awash with links about this; here's the NYT story. There's really two instances of misconduct here. The first is the responsibility of United Airlines: they overbooked a flight, then wanted to force off the plane seated, ticketed passengers to make room for flight personnel who needed to pick up a connecting flight. Whoever at United made this idiotic decision should be fired (their name will no doubt come out in the ensuing litigation--unless United settles very quickly, which they probably will). The second is the responsibility of the police, who did what police often do when they don't get what they ask for: they become violent. The police were presumably told that this passenger had to deplane; they probably didn't even know why. The lawsuit against the City of Chicago--which knows a lot about defending against such lawsuits--may shed some light on what transpired--though, again, the presence of a video of what happened means a quick settlement is also likely.
The doctor who was manhandled here was already on the phone with his lawyer when ousted. I assume we will hear about the lawsuits within the next 48 to 72 hours.