It's just a complete waste of time to think about these kinds of things. Some humans malfunction and do this kind of thing. It happens quite rarely, and there is nothing that can be done to stop it. One's probability of being harmed by the occasional insane murderer (or terrorism for that matter) is microscopic compared to so many other things that we can actually do something about.
I understand what you're saying, but really "there is nothi that can be done to stop it"? What about outlawing firearms for civilian use, or even just not letting the assault weapons ban lapse? And don't tell me every angry/depressed yuppie kid who was going to go on a rampage when guns were easier to get than a drivers license is going to negotiate some black market underworld he knows nothing about to get his guns.
As an example, look at the 2009 Chengu bus fire: http://en.wikipedia.org/wiki/2009_Chengdu_bus_fire. No guns were involved, only gasoline, but 27 people died and 76 more were injured or maimed in this murder suicide. Or, look at this event from 1988 where 31 people were killed using dynamite: http://news.google.com/newspapers?id=7EJYAAAAIBAJ&sjid=3...