Inside a dark theater, with smoke and people running all over and taking cover, even a trained SEAL team or SWAT member would have a very difficult time trying to figure out exactly who the shooter is and where he is.
On top of that, lets say a Legally armed citizen decides to return fire.....now who is the active shooter in a dark theater. Even if a Legally armed citizen takes out the bad guy its a good bet the legally armed citizen (LAC) is going to be arrested (roughly) until someone figures out who did what.
Taking out a BG in an active fire zone with hundreds of people around is a tricky thing. WHEN the cops show up they may just think YOUR the BG. Im sure there apology will be sincere, but you'll be just as dead as the BG.