Because he hopped up on stage? Come on, how can you 'review' Aurora and conclude that? You have to be assuming someone with the training/experience to have nerves of steel, identify a muzzle flash, and then fire right where he/she needed to to hit the target, all in the dark. Again, not an expert, but that has to be luck.
I was focused mainly on the hypothetical in-the-dark theatre, but it can be expanded. Let me put it this way: the more armed people, the more likely chaos will lead to extra casualties, the lower the probability that a good shot with a gun will get the bad guy in time. The dark exacerbates this.
Would there be good cases? Yes. But I can only think there would be a lot more of worse cases if a small group of heavily armed people in an enclosed space - especially if it's dark - has a contingent of armed people at various locations in the crowd, who all just start shooting at the person they think is the one with the gun.
(On that note... yet another danger: good guys with a gun mistaking good guys with a gun for a bad guy. Add more: cops showing up and seeing a shootout. What then?)
The good guy with a gun thing only works if it's a small setting where the target is neutralized quickly AND before the police show up. The more people/elements you throw in, the worse the recipe.