Glenn Reynolds points out that "gun-free zones" only disarm law-abiding citizens.
One of the interesting characteristics of mass shootings is that they generally occur in places where firearms are banned: malls, schools, etc. That was the finding of a famous 1999 study by John Lott of the University of Maryland and William Landes of the University of Chicago, and it appears to have been borne out by experience since then as well.
In a way, this is no surprise. If there's someone present with a gun when a mass shooting begins, the shooter is likely to be shot himself. And, in fact, many mass shootings -- from the high school shooting by Luke Woodham in Pearl, Miss., to the New Life Church shooting in Colorado Springs, Colo., where an armed volunteer shot the attacker -- have been terminated when someone retrieved a gun from a car or elsewhere and confronted the shooter.
Policies making areas "gun free" provide a sense of safety to those who engage in magical thinking, but in practice, of course, killers aren't stopped by gun-free zones. As always, it's the honest people -- the very ones you want to be armed -- who tend to obey the law.
One of the best ways to protect society is to put more guns into the hands of law-abiding adults.