One excuse that I saw repeated over and over again was that if anyone else in that theater had been armed, it would have been "chaos, with bullets flying everywhere, hitting more innocent people". Sure, that is possible. I don't think it's likely, though.
A decent person isn't going to start firing blindly even when there's an active shooter in the room. They just don't do that. "Know your target and what's beyond it." It isn't going to be hard to tell which person is trying to kill others (unless you have "law enforcement" training, that is): he's the one taking aim at the crowd. The people on your side are the ones taking aim at him.
Sure, there is always the possibility of hitting the wrong person in such a situation- that's why you would have to decide for yourself whether you can take a shot or not. If not, you are no worse off than you would be if you were not armed (until the cops show up and kill you), and an opportunity still might arise where you can do some good.
Some worried that an armed person would have just attracted the shooter's attention and been targeted as soon as he showed himself. Possibly. But then that gun would have been available for someone else to attempt to use against the bad guy.
Look, some things are just going to go badly. It's "life". Sometimes, in spite of your best efforts, you will not make a positive difference. If that's enough reason for you to never try, then... I have nothing but pity and contempt for you. You can always find some excuse to be a sheeple if that's all you want to be.