The decline in violent crime is kind of is relevant. It didn't just happen, in the US at least, it was the result of years of effort, the investment of significant amounts of money, improved law enforcement, and mostly, recognizing where the problems were and confronting them.
And still things aren't what we'd like. They are much better than they were, though--

As far as the Aurora Shootings, call them mass murders or binge killings, or whatever you like--because they are the acts of deranged, but often very intelligent minds, they are unique and infrequent. So it's difficult to anticipate them, and nearly impossible develop preventative strategies.