When the airline asked him to leave and he refused he became a trespasser on the plane.
Prove it. Show me the law or the case in which this precedence was established. The airlines own policies appear to indicate that Mr Dao was within his rights to remain seated once he had boarded and seated.
Things are better if you make sure your customers don't have to make a dumb decision, but it was his dumb decision to stay and not get off.
Things are better if nonviolent airline passengers are not subjected to a beating that causes broken teeth, a broken nose and concussion.