All these commenters giving legal advice about this airline passenger sueing the airlines should go back to flipping burgers. The airline ticket is part of a contract, and there are provisions in that contract that allow the airline to bump passengers. There are federal rules and regulations that allow airlines to remove passengers. The level of force used was a direct result of the passenger refusing to comply with the lawful orders of the flight crew and security personnel. The information about the passenger’s history and mental state is directly relevant because it explains why he acted as he did and why force was needed.
Once this passenger became belligerent the airline had no choice but to remove him while the plane was still on the ground. If the plane had taken off and the passenger got belligerent with the flight crew in midair then all these people shedding crocodile tears for this guy would be screaming at the airline for not removing him before takeoff.
Bottom line is this guy brought it upon himself when he decided to ignore the lawful (even if unpopular, at the time) order of the flight crew and security.