Today I was musing that one of the aspects of these kinds of horrors that makes them more likely to distort most people's perceptions than "everyday murders" is not their magnitude so much as their relative unpreventability. What I mean is: Most murders, as gopower points out, are "crimes of passion". The murderer is well-known to the victim, who may even have tried to escape their assaults in the past. To the extant that we can refuse to associate with violent, unstable personalities, we can reduce our chances of being victims of such a perpetrator. But we have much less control over who our colleagues and neighbours chose to associate with, and with that uncertainty and loss of control comes a tendency to exaggerate the risks.

Now that I've laid it all out there, it looks like a pretty banal realisation. But that describes pretty much all of my reactions to the massacre.