Regardless of his past criminal record or difficult, belligerent demeanor it still does not give United Airlines the right to physically assault him.  None of that is relevant to what took place.  The man was not a threat any other passengers, the crew, or the plane itself.  He was no different than any other passenger on that flight.  If United wanted him give up his paid for seat they need to make it worthwhile to Dr. Doa to do so.   They had no right to physically assault and remove him.