The police had no option, he had to come off peacefully or forcibly. He chose the harder of the two. The scene and injuries were indeed very unfortunate. If he wasn't in a confined space surrounded by people he might have been tasered or sprayed.
Are you kidding?? He was a law abiding passenger sat in his seat minding his own business. So it is acceptable to assault him because United have made a mess?? Let them (United) stew in their own juice.... A situation entirely of their own making...