Yeah the grabbing for the gun excuse came out very quick from the police, unfortunately for them the point at which he decides to kick a the man in the face and then stamp on his head is on video and and as he's face down on the ground even if this had previously happened it wouldn't justify his actions.
If someone gets shot trying to grab an officers gun, absolutely fair enough.
If they manage to subdue that person without shooting him, they don't then get a free hit so it's largely irrelevant to the actions that have been caught on camera such as making people for filming and then illegally searching people to try and seize their phones