When mass killers meet armed resistance is a blog post you can read here .

It reports that there are several incidents where nutballs set out to kill a LOT of people but were stopped short of their goals by armed citizens well before the police were able to get there.

Basically, the idea is that there would be even MORE Columbine massacres if it weren't for some brave souls who brought their guns to school (or work, or shopping) that day.