The Department of Homeland Security (Houston) produced this video. Warning, you might find the depiction to be disturbing. There’s a bit of FUD in the beginning, particularly… “occasionally, life feels more like an action movie than reality”, because “occasionally” is hugely overstating the frequency of these sorts of events. The odds that you’ll ever use this advice are vanishingly close to zero.
This video actually does a very good job of detailing what you should do, in the very unlikely event that you’re witness to a mass shooting event. Run, hide, and fight. In that order. Regardless of whether you are armed or not, you should run first, hide second, and fight only if cornered and have no other option.
You are not a trained first responder. There may be other, untrained, non-first-responders at a shooting event. Some of them may be responding with their own gun, and you don’t want to shoot a defender by mistake. Joseph Zamudio is a licensed CCW permit holder who was present and armed at the Gabrielle Giffords shooting, Mr. Zamudio didn’t use his weapon because of a split-second decision and uncertainty that the target he was engaging was in fact the first shooter.
Remember John McClane, in the first Die Hard movie? What’s he do for the first half of the movie? Run, hide, call the cops. “Why didn’t you try to stop them, John?!” he asks himself. And then answers, “Because then you’d be dead, too, asshole!” It’s only after the colossally inept movie cinema cops start being stupid that he takes matters into his own hands. In real life, the cops aren’t quite so colossally inept. They’re almost certainly going to be better at handling the situation than you are.