I just read this guy was also wearing a throat, crotch, leg protectors and a Kevlar helmet. (not a riot). So it would have had to have been a face shot.
I’m not impugning your marksmanship but to get a face shot or even a head off under fire, with all the confusion of smoke/teargas, in the dark, people screaming, running, dying, fire alarm blearing, and the movie still playing would be a remarkable feat.
Now all that being said, I do believe that if a person or two would have been able to return fire, it would have given more people a chance to escape. It would have made the shooter concentrate his fire on them. Or given the way he just surrendered to police in the parking lot he may have just retreated once he saw he was under return fire. But you do have to remember something they taught me in the army. There is nothing harder to stop than a determined individual bent on carrying out their plan. (just look at what Obama has done.)
Concealed weapons will not end things like this from happening. But they would most likely change the outcome and probably save lives.