This is like OODA Loop: The Video.
He did try to ditch the gun, but it was akin to a soldier trying to throw up his hands and surrender after a live grenade just landed at his feet. Police have reactionary gaps just like everyone else, trying to surrender won't do you any good when they're already halfway through the trigger press because you threw open the door and stormed out with a gun in your hand. Tragic, but the police are not at fault simply because they failed to use superhuman means to save you from your own bad choices.
He wanted to play tough guy with a gun.
I guess the scorching heat in Arizona screws with reactionary gaps. Ask Daniel Shaver.