As sad as it is, there is not much we can do about it... Banning guns, in general, is out of the question, and by the time we figure out who the person is that's capable of committing a mass shooting, its too late.
Had a kidnapping yesterday morning in our small town, it was a ransom deal. In the end they let her go (12 years old) This reinforced my staying outside to wait for the bus with my kids.
But, how will we make sure, 100% that the "unstable ones" won't have access to guns? As a matter of fact, how do we determine who the unstable ones are until they do something to show their instability?
If you take people's guns away, and then ban knives, then criminals will just beat you to death with golf clubs. Do you really want to be killed by a bunch of golfers? Will school shootings be replaced by school clubbings? Will kids post ominous warnings that they're "going clubbing" while posing with their 9 iron? Or worse, they could just use boards with nails through them. The Simpsons warned us about that. Two highly advanced aliens landed in Springfield to take the place over and were chased off by people with boards with nails through them. No matter how advanced your technology, you can still be killed by a board with a nail through it. The only thing that can save us from golfers and boards with nails through them is a gun. Think about it.