Can’t halt massacres until they happen. How do you prevent something like what happened in CT?
You can’t prevent massacres from being attempted, but a properly trained person could halt the kind of thing that just happened. Two or three dead instead of 28.
You're a legally carrying, firearms trained adult
You're in a school as (whatever) and you hear shots
You follow your ears, surmise the situation and shoot the shooter.
It's a better plan than calling first responders 5 minutes away, that take 20 to arrive and stand around after they arrive.