Yeah but then there's all that paperwork afterwards ya know? Such a pain.but the only 100% sure method to keep him from grabbing the gun would have been to step up, place your barrel against his head, and pull the trigger.
From what I could see in the video the officer's gun didn't clear the holster which is always a good thing in a situation like that.He made the decision to go after a firearm (thereby freely accepting the consequences of that decision) and the only way to ensure the safety of innocents in cases like this is to neutralize the threat ASAP.
Better to have tasers and stun-belts than actual firearms in a courtroom. There are certain career criminals who will stop at NOTHING to prevent going back to prison again.