I also wonder, if the situation is how it sounds, if even a bystander with a gun could have really done anything
If I put myself in that food court and saw a large group of people arguing, and one pulls out a gun and starts shooting, then chaos ensues, I feel like all I could do is take cover and/or run.
There would be too much risk hitting a friendly in a crowded mall. It seems like there was only a few intended targets so it wasn't an indiscriminate shooting situation like a terrorist event. And after the first few shots the beef was likely over so there wouldn't have been any other imminent harm to others
The other thing to consider is if you're carrying and the police have to detain and question EVERYONE that was there. How much of a hard time would someone get that's legally carrying at the time but they "shouldn't" according to the mall signage? It's not something I personally dwell on, but how much would that suck to wait forever for police to have to deal with a legally armed citizen that was there during an active shooter event and no suspects?
This is why I avoid large crowds if I can and I certainly only go to the mall during the day in the middle of the week