If I was Dr. Dao, I would have been pissed as hell. But I also would have gotten off the plane. Because this HAS happened to me before - previous flight was cancelled, then got on standby on the next flight, there was a no show, so I was sitting in my seat for at least 5 minutes before the no show showed up and they made me get off the plane.

The Dr. refused a lawful (if highly fucking annoying) request, and then when they tried to force ably remove him, he started screaming. He was not "beaten" or "physically attacked".