Both the passengers and the videos depict Dao as firm but calm right up until the thugs started in on him. He didn’t throw tantrums or act immaturely. He simply stated his need and determination to remain on the flight.
Till the Police officers did what any reasonable person would have realized they were going to do.
Once it became clear that the police had been called, it was time to comply. When the police are clear in their intent to evict you, you are going to get evicted even if it takes force to do so.
The man could have gotten his neck broken. He could have gotten a brain hemorrhage from the injury. Why would anyone risk that by resisting the police for no other reason that they might miss their flight?
It is foolish to resist "the thugs" for no better reason than this, especially if "the thugs" have no intention of hurting you so long as you cooperate.