I've been reading about the Virginia Polytechnic shooting spree, which prompted an hour long discussion in my Risk Mitigation class. You really can't shut down the world because someone might snap. I think that noticing people who are alienated and feeling enraged and disconnected from humanity and at least starting a conversation with them can diffuse a lot of terrible behavior. Aside from those measures, you're more likely to get struck by lightning than get shot by a psychotic mass murderer. I think if you spent the money that would be spent on metal detectors and served free breakfast, more deaths could be prevented. That said, they really needed a better incident response plan at that school to reduce the amount of damage a random shooter could cause. Couldn't someone have pulled the fire alarm?
In my reading on the topic, I found the writings of Michael Welner, who studies these kinds of individuals who go off the deep end and conduct a catastrophe. He has developed a survey which ranks crimes as to whether they fit the community standard of "depraved"or not. It is becoming the legal definition, and he the foremost expert. His site explains it better than I can. The goal is to make the sentencing guidelines less arbitrary when people are convicted of "heinous" or "evil" acts. I found it mega interesting. http://depravityscale.org/