Dao was fine until they brutally slammed his head into the armrest.
My recollection of the video is that when they finally pulled him out of his seat, he fell onto the arm rest. I do not think the police officers "slammed his head into the armrest", I think he fell into it after they pulled him from his seat.
This is the sort of thing you can expect to happen if you actively resist security personnel on an airplane. This is why it is a bad idea to do it.