You brought up Aurora and so did Bill O'Reilly last night in the interview with Costas.
The movie theater is dark and a gunman starts firing.
Law abiding citizens are panicked (understatement) to begin with and have to make a split second decision.
How do I know if the first law abiding person firing at the gunman isn't one of his accomplices?
Now the second law abiding citizen sees two people firing weapons and it's time for the terrified person to make a split second decision and decide who are the bad guys and who are the good guys.
And so on.
What could possibly go wrong?