Addition to some of the arguments for others carrying, you have the deterrent factor. It was a dark theater, so if multiple people had pulled guns, he wouldn't have been able to specifically target them all and try and kill them first, especially not with the clouds of gas in the air distorting his view. If he saw multiple people with guns pointed at him, or was shoot by one of those guns, it very well may have ended the shooting spree sooner, either by taking him down (if he was indeed wearing a tactical vest and not a bulletproof vest), or by scaring him off if he's the one getting shot at and not the one doing the shooting.

Yes, others could have been hit, that will always be a possibility when guns are involved.