Personally, I think USA would benefit from taking a long hard look at how European countries deal with issues such as these, so it should care what Swedes say. Maybe, then, it could see itself more clearly in a brighter light. It is not alone in the world, and it has no monopoly on what is right.
I think Tom's suggestions are probably the best on offer so far. Certainly poverty is an issue, because I am sure that most armed crime is committed by the poor and hopeless. Education, too, is important, and I would suggest that smaller schools would go a long way to helping, too. Where students number a few hundred rather than several hundred or even thousands, they become impersonal and potentially hostile places where all forms of bullying and petty crime can flourish unchecked: smaller schools can identify such behaviour quicker and stamp it out easier. Also, it is easier to identify with a smaller school and build ties of loyalty. This is character forming. I would suggest also that smaller schools would be able to produce better levels of education, so fewer people, even poor people, would feel quite so helpless and hopeless.
But none of that stops the "crazies". And they must either be prevented from assassinating their school chums, or they must suffer punishment for doing so afterwards. If prevention is seen as better than cure, isn't the first step to make it nigh on impossible to get hold of guns?
Sure, it means that a "freedom" is sacrificed: the freedom to do what? And does that amount to a hill of beans?
TYWD