To be fair to the Officers while he was laying on the ground he still had the gun in his hand and was said to be trying to lift it. You can't see it all in the video. He was high on PCP which is a drug that makes people very hard to take down by any means. Unless you have dealt with someone on it you truly have no idea. They believe they are invincible and their bodies respond accordingly.
My thoughts too. I wonder if there is another video with a different angle.