This is one I just saw, but have seen coming for a long time. Those who decide to kill lots of people tend to go where they can cause the most carnage with the least obstruction and when the cavalry does arrive, they kill themselves. So if you want to invite a crazed mass murderer to your place of business or school, prohibit carrying guns, and the true crazies show up to commit suicide and take as many with them before they kill themselves.

Of course that means that those places that allow people to defend themselves will have fewer crazies coming to them.