All the worst mass murders happen in gun free zones - all the school shootings that were stopped quickly were in areas that allowed the armed citizen to intervene.
Well, now there's an unfounded claim.
For example, the aforementioned Mr. Whitman took a large number of lives, despite the fact that both police and armed civilians were shooting at him.
As a matter of fact, I can't recall a single case where school shooters had been thwarted by armed civilians -- even though some of those shootings have occurred in places where personal firearms ownership is common.
The one in Pearl MS ended when a teacher ran out to his car, got his gun and put a bead on the shooter, bringing him to his knees. He held the gunman there until police arrived. It's possible his having the gun in his trunk was against the law, but I'm not sure if that 1000' from campus law had been enacted in MS.