Someone who isn't going to follow the order of gate staff, cabin crew and police don't really fit the criteria for flying anway.
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.
None of us like being "told", but sometimes you have to just cop it on the chin. Obviously lack of sleep/long day etc can make it harder.
United did what probably every airline in the world have had to do at some stage.
It never ceases to amaze me what Police, Fire, Ambos put up with in their day to day duties.