I haven't read through the other posts yet, but here's what I can say to the original question:
I work in law enforcement, and have been training for a decade to counter active shooters in the real world. To officers on the street these days it is most definitely a real threat that we may someday face. Being that I work in Colorado, we seem to have a disproportionate number of these shootings for some odd reason (Columbine, Safeway, Platte Canyon, Aurora Theater, etc).
Anyway, my first priority is always my family. I'm not apologetic about that fact. If I've got my people with me when an active shooting starts, saving them is my primary function. Whether that means sneaking them out the back, pointing them in the right direction, shooting my way through the bad guy, or providing armed cover for them while they take a safe position, my goal is still their safety, first and foremost.
Once my people are in relative safety, I'm going hunting. There's no way in good conscience that I could allow some psychopath to kill innocent people without acting. I'm armed when I'm out, and I'm plenty capable, trained, and experienced to take on such a task. It's an ugly task, and it is potentially a very tough and dangerous tactical situation to be in if I'm just carrying my CCW gun, and a bad guy is armed to the teeth for a mass shooting. But, I hate those types of cowards, and I've built a career around protecting the weak from the predators... I'd pursue that predator to the best of my ability.
As a very serious word of caution, should any of you ever find yourself in such a dire situation, please be very aware of the fact that the bullet you take could easily come from a responding officer. When we move in on an active shooter we move in fast and hard, because of the fact that lives are being lost with every second of delay. In doing so it could be easy to mistake an armed and civilian-clothed citizen (or even an off-duty officer) for a suspect. If you ever find yourself in such a scenario it will be absolutely paramount that you keep your head on a swivel, stay mindful of the changing environment, and don't give responding officers ANY opportunity to mistake you for a bad guy. In other words, if you suddenly hear a command of "drop the gun", you better realize that your life may hang in the balance depending on your response to that command. We just had a class on this issue recently, and it is truly sad to hear about the off-duty police officers who have been accidentally killed over the years as a result of mistaken identity during a critical incident. It can happen to you if you're in such a situation and aren't aware of that threat.
I have a badge with me when I'm off-duty, and I'm still very concerned with the sad reality that friendly fire may be the greatest threat I face should I ever be required to use my gun in a public situation. For my part of it, I'd probably try to stay holstered whenever I could, especially if I knew that responding on-duty officers were making an approach. Anyway, sorry to jack the thread a bit, but that's something everyone should consider if they are taking the time to consider how they'd respond to this type of scenario.