It's pretty much a mantra by now used to defend wildly unlimited open carry of even the most lethal of semi-automatic and fully-automatic killing machines:
"The only that stops a bad guy with a gun is a good guy with a gun."
This idiocy is used to defend the idea of as many people packing heat as possible to deal with, you know, shooters walking into an elementary school and mowing down kids and teachers. Unfortunately, that maxim has an absolutely fatal flaw, which I will now explain:
If everyone can carry, there is no way to tell the good guys from the bad guys until it's too late. That this even needs to be explained is depressing beyond imagination, but let's give it a shot. Pun intended.
"If everyone at that school had been armed, that kid would never have been able to kill 19 children."
Really? And how does that work, exactly? Because if everyone has the right to strap an AR-15 to their back and wander around freely, then how do you tell the good guys from the bad guys in the first place?
So some guy in full camo and an assault rifle enters a school. Is he there to cause mayhem? Or maybe, because he has the legal right, he's just come from deer hunting and is there to pick up his daughter. How can you know the difference? What makes him an obvious "bad guy"? He enters a classroom full of kids. Is there any cause for alarm yet? How are you going to know, until he unslings his AR-15 and starts shooting kids? If everyone is free to wander around with a fully-loaded assault weapon, then there's nothing you can do when someone so equipped walks into a school. Or a church. Or a synagogue.
Until they start shooting. By then, it's kinda too late, but I assume you figured that out by now.