The thing that struck me first about the arson in the South Korean Subway was that it could happen anywhere. A guy, probably a nutball (although I wouldn't necessarily trust the South Korean press not to cover up an act of political terror), a milk carton full of explosives, and a lighter. In any remotely free society, there is no law enforcement, no gun control laws, no information database that can stop it.
Not to say that it'll happen here; it hasn't yet, and I sure hope it doesn't. In our society, most people who might get to that point of madness are treated and/or confined. Also, societal norms still function at least to place the killing of innocent people outside the proverbial pale, particularly in large numbers. But it could happen at any time. And, as it is written, the beginning of philosophy is to accept that anything that can happen, can happen to you.