Police are the ultimate “good guys with a gun” — we train them, we employ them specifically for this purpose — yet even they can’t (or won’t) protect us in every situation, that’s an argument against relying on good guys with guns.
But I understand that by “good guy with a gun” you want to suggest that we rely for our safety not upon police, but rather upon strangers, who may be decent people at heart, but do not have the training a police officer might get and aren’t obligated to defend you at all. Citizens can turn tail and run from active shooters, regardless of how armed they are, and frankly probably should. I don’t know any police officers who would prefer to run into the fog-of-war of a multiple-active shooter situation than one with just one shooter.
Let’s go with an even more on-point example. Rare, but illustrative.
Let’s assume that we have a mass shooter on the loose, who’s already killed 2 people, but before the authorities arrive, he’s stopped cold by a “good guy with a gun” who’s both well-trained enough to hit his target and courageous enough to actually do it.
If you’re a gun advocate, then this is your ideal situation! An average armed citizen standing up courageously against evil, just like our Founders intended!
Problem is, after this riveting clash of good-and-evil, we are still left with 3 dead bodies — dead from gunfire, than we would have had in a society that never allowed that situation to happen.
(Of course, most mass shootings do *not* end in this way. They most likely end with on the one hand, the shooter offing himself and taking the coward’s way out, or on the other, with the SWAT team.)
What the “good guy with a gun” scenario shows is that even in the best of circumstances, gun advocates have bodies to answer for. They always do.