I'd like to point out that a "good guy with a gun" INCLUDES THE POLICE!!
And armed citizens, and off duty police.
The reason you don't see many (any, really) of these shootings stopped before they start is simple. The attacker(s) ALWAYS have the advantage of complete surprise. Literally, no one but the killer knows what is going to happen until the shooting starts.
Then, you have to have a good guy with a gun THERE, in a position to respond, and then, they have to respond, effectively.
Even though there are "more guns carried today" than ever before, the odds of someone having one at the right time and location to respond to a shooter is extremely small,
And if those tiny odds are beaten, and there is someone armed who could respond, they have to actually respond, AND then that response has to be effective. And NONE of that can happen until there is something to respond to...
SO, you can't stop these things before the shooting starts, and then, in effect, there has to be multiple "lottery wins" for an armed citizen to stop the attack. The police are in the same position, but a bit better off, as they are "they" and not "you". Responding in numbers, with some idea of the situation gives those "good guys" better odds than the single armed individual, civilian or (off duty) police.
There have been instances of single armed responders stopping mass shooters before the police force did. There have also been failures. Someone (usually the police) has stopped almost every one of them, eventually. And this includes those killers who killed themselves, when someone (not always the police) frustrated their plan.
There has been one, perhaps more, where the shooter wasn't stopped at all, and finished their attack, THEN got stopped afterwards.
No one in New Zealand legally buys a gun with out police approval. Unless there is some kind of massive breakdown in the system, no one who is considered at threat (at the time of investigation) get approved.
Time after time we see these killers pass every required test, EXACTLY THE SAME as the rest of us, and then, later go on a killing rampage. BECAUSE THEY CHOSE TO!
The mind of man is as trackless as a bog at midnight...
No one knows what evil lurks in the hearts of men, and the Shadow isn't telling...
I hear this over and over, "the killer bought his guns legally!!!" and the response is to ban certain guns, and make buying any guns more difficult.
Never once do I hear how the killer wasn't a killer, or any kind of recognizable threat when they bought their guns. They were the same as the rest of us, citizens with legal rights who had committed no crimes.