I have traditionally found stories of 'good samaritan' gunmen who take out active shooters largely to be annoyingly frustrating red herrings.
In these rare scenarios, yeah, it is good someone stopped four people shot from being more. But mass shootings make up only a small portion of the overall gun and gun violence problem in this country. Not to mention that fact that most homes don't have guns. Most people don't want to own or carry guns. So, when you get these stories, all it primarily does is manifest these fringe firearm fantasies for gun activists to latch onto as proof of what a well-armed citizenry can accomplish. Never mind, of course, the obscene access to guns our adults and children have, the number of suicides committed by them, the majority number of crimes that happen with guns that are unrelated to mass shootings, etc...