I just think the teargas would have seriously blinded you, making it next to impossible to aim in the dark.
Not as much as you would think. Trying to breathe clean air takes a higher priority, your mind will make your eyes deal with it until you can get to fresh air. And shooting with a gas mask on isn't exactly easy either.
Unfortunately, the media won't inform everyone of things that could have been done to limit the casualties. Concealed weapons or not, a room full of 200 people and only one shooter. If everyone would have charged the shooter, he would have been trampled before he had a chance to shoot 71 folks. Of course, if everyone had the mindset to be that pro-active, they would probably be carrying.