So, we know of at least two armed citizens at the scene.
One with his gun drawn watching the struggle to subdue the killer and ready should stronger measures be needed.
And the one in the video who had his hand on his gun and the safety off and totally ready to shoot if necessary. He saw the other citizen with a gun but since that man wasn't threatening anyone with it, chose to disarm him rather than open fire.
Not a bad accounting for those two.
The only wild shooting was from the madman. The citizens where there, able, willing and prepared to take responsible action with their weapons.