I haven't had a chance to read Nassim Nicholas Taleb's book "The Black Swan: The Impact of The Highly Improbable" yet, but I thought that Niall Ferguson's piece in the Telegraph last week, on it and how it relates to the coverage of the Virgina Tech massacre, was pretty eye-opening. The key insight is that there is a human cognitive bias which prevents us from appreciating improbable events - we tend to conflate improbable with impossible - and we are also naturally predisposed towards creating narratives or "retrospective predictions" for such events. It's easy (or rather facile) to see *now* the sequence of events leading to Cho's rampage and it's an easy trap to fall into to (incorrectly) assume that this sequence should have been obvious before the massacre.
By chance a similar discussion about a "family annihilation" has been taking place here in Ireland: Adrian Dunne killed his two young daughters and his wife (the official line is that she wasn't herself involved with the planning and implementation of these killings) before hanging himself. Most of the debate centres around what could and should have been done by the authorities to prevent this tragedy: Dunne had visited a funeral home shortly before and had ordered four coffins and given detailed instructions for the funeral in the event that an "accident" took place. The popular, and in my view incorrect, assumption seems to be that this event was utterly predictable given the (now) compelling narrative leading up to it.