[quote]It almost makes me wonder how many of those people might be capable of such an act.[/quote]

All of them. Everybody. People are not a nice creation, once our illusions of order are taken away. The only people 'Incapable' of a mass killing are the ones with poor aim. 

But look at the bright side. We've gotten [i]really really goddamn good[/i] at suppressing our violent tendencies. We've come a long way since Eye for an Eye. Heck, even people in the army are more likely to die in a motorcycle accident than they are to get shot. We've got an impressive society, and these breaches only emphasize that fact. Human nature is ugly and we have made ourselves awesome.

EDIT: This may seem in poor taste. Let me know if I've been a little too blaise, I'll..... ech. My instinct now is to take it down but I think I'll leave it here until/unless someone asks, and then I've got no qualms deleting this post.