From the attacks of 9/11 to Monday's school shooting, after every mass murder there is an overwhelming urge to "do something" to prevent a similar attack.
But since Adam ate the apple and let evil into the world, deranged individuals have existed.
Most of the time they can't be locked up until it's too late. It's not against the law to be crazy — in some jurisdictions it actually makes you more viable as a candidate for public office.

"Let's Make America a 'Sad-Free Zone'!" (18 April 2007).