"Why not shoot in the air once to stop him?"
Maybe because LEO are trained to discharge their weapons ONLY when using DEADLY force is neccessary.
They also don't trick shot the guns out of criminal's hands or "kneecap" them so they don't run away.
He 's been watching too much TV.
I believe that "he was going for my gun" is a convenient excuse for LEOs when they screw up.