What order was he disobeying? They pepper sprayed him and dragged him to the flor, he didn't have a gun in his hands, no danger at all, so why shoot him? He wasn't grabbing anything, you see his hands are free in the video.
And not behaving peacefully (who would when you are pepper sprayed and are hitting you in the head) shouldn't (in a decent country) be a reason to be shot.