I suspect the perp just walked up to the people laying on the ground and shot them in the head or shot the ones who were scrambling for the (apparently locked) doors in the back.
If a half dozen people had charged him, he could not have taken aim at any of them. He would certainly have hit a couple before they swarmed him, but in the end he would have been tackled and disarmed.
It’s like a bayonet charge. The enemy is caught off guard by charging troops. They shoot wildly when the target is moving rapidly towards them and even more wildly when there are several targets moving in. At that point they are as scared and panicked as the people they are shooting at.
But war tactics are not taught in public schools where even the mention of a gunman can get you expelled.
IIRC, Chamberlain’s bayonet charge at Gettyburg saved the Union flank and resulted in Robert E Lee’s loss. Essentially, they ran out of ammo. So they ran at the enemy instead of away from them. Scared the bejeebers out of them.
But, I agree with you, that a half dozen or a dozen would have easily overcome that guy.