I really don't think random dudes with guns trying to be John Wayne is going to help anything.

Lets say there is a mass shooter in a school, and two people with guns go and try to hunt him down, how do we know they will make the right choice and kill the killer?  What if they think the other "defender" is with the killer?  What if one of those defenders misses and it just turns into an all out gunfight with tons of innocents running every way.  You aren't Clint Eastwood, you aren't some badass movie hero, you are a dude with a gun.  

As for you thinking that we need to shoot these people to prevent others from doing it, that wouldn't deter anybody because these people do shit like this already expecting to die, most of them want to.  Wouldn't it be more of a punishment to not give them the death by cop they are almost always trying to get?