That's not an armed civilian and not part of the audience, but a particular trained person whose exact job it was to secure safety. So the incident is no argument for civilians to run around with guns, but the opposite. It's an argument that those situations are best solved by the police or other particularly trained forces.
Tbh I couldn't find much of anything as far as theatre shootings. The case I linked is by far the closest thing. Sorry I couldn't find exactly what you were looking for. Here's a list of mass shootings from Columbine forward.
Here's a list of civilians protecting themselves with firearms if you're interested.