I did actually think about that, but I wouldn't call them a "good guy with a gun". I guess police are dubious too but I think the ones using that saying would count them.
I think nobody is consistently only good or only bad, but we can say all shooters stopping an attacker is good in the moment they stopped the attack, even the attacker deciding to off themselves.
The "good guy with a gun" trope in US gun control discourse is based strictly around a civilian who carries a firearm with them, not police or security whose job it is to carry a firearm and keep people safe. That's 12/433 people, not 28%. 15/433 at most if you count the off-duty cops.
When people talk about "good guys with guns" to stop mass shootings, it's a bullshit way of deflecting from the actual problem, instead going in the opposite direction of the solution by saying even more civilians should be armed.