(Image by okaygeek.com)
People say that any hypothetical defender at the Aurora theater would have run the risk of hitting innocent bystanders. The risk existed but consider how typical theater seats rise rise towards the back. It’s the classic “second rank fires over the heads of the first” musketeer arrangement. A perp near the screen would have been on the receiving end of all armed theater patrons simultaneously…had any armed people besides the attacker been present.
As for the aiming in the dark: gun lights exist for a reason. Moreover, in a smoky atmosphere, laser beams would have become visible and pointed to the attacker most prominently. Green lasers in particular provide enough scatter to light up the target fairly well. Worst case, the combination of screen light and muzzle flashes would have provided some idea of where to shoot — and the viewers eyes had time to adapt to the darkness.
So it would have been a tough fight but far from impossible — certainly not on the level of Beslan school takeover.