In Aurora those citizens were trained and some had seen combat.
Several of the people in the theatre that night were combat veterans. At least one was a Marine who had served multiple tours in Iraq and Afghanistan and had seen combat in both countries. In other words, a man who knew combat situations, had MOUT training (and had likely been involved in urban combat). In other words, one thing which would have happened in all likely-hood, was that the shooter would have been taken down rapidly by people who knew how to do just that. It could have saved lives if they'd been armed that night.