Why didn't a good guy with a gun just shoot them ?
Someone please answer this question, there's NO way every mass shooting in Texas happened where someone didn't have they guns.
People who carry guns are typically the kind of people who believe that firepower is what matters and are scared of getting in a situation when they are outpowered. That's why they carry - so they can be the guy with the gun.
Then when there's a big shooting where the other guy has a bigger gun or is clearly a bigger savage, they feel outpowered, just like they would feel if they didn't have a gun at all. So they take cover or get the hell out of there.
Or, if they're semi-rational and not just cowardly, they see that it's a confusing situation, people running everywhere, they might not even ID the shooter right away, when they do they see there isn't a clear line of fire or they see that any shots they miss could go on to hit other civilians, and they realize it's just not a safe scenario to fire in. The other guy is a savage who doesn't care who he hits, while you have to worry about all the people you are going to it, so he has a HUGE advantage. Plus, other "good guys with a gun" or the police might be about to show up, and how will they know whether to shoot the other guy or shoot you?