Why not? Civilians are almost always the first responders to a crime scene, and if you have civilians who are armed within the immediate vicinity, they are fully capable of killing the gunman before he can do his full intent of damage.
The United States has some of the highest rates of gun ownership, but random mass-shootings almost never seem to be stopped by vigilante gunmen. Why is that? Because people are less likely to pull out weapons when law-enforcement is en-route to the scene and expecting to come and shoot an ordinary-looking person with a gun. Nobody wants to try and be a hero just to be killed by the cops while doing it.
