So where do we go from here? How do we prevent this from happening again? It seems that there are two schools of thought (generalizing obviously): 1. Disarm everyone, and 2. Allow everyone to carry weapons. Regardless of which side you fall in, neither work perfectly unless they are complete (i.e. all weapons are gone thus criminals don't even have access, or everyone is armed and no one has the upper hand). The problem with both absolutes, is a deranged person will always find a means to carry out their ill will, whether that's a gun/knife/driving a car into a crowd.
The safest computer is encased in concrete, and buried 6 feet underground. Much in the same way, the safest society would have each of us locked in a room, with no interaction. What we have to figure out is this: How much liberty do we all give up, to limit the devastation of the senseless acts of a few?
I think these are actually two extreme positions to which almost nobody seriously subscribes.