In an essay in the October 19 edition of the New Yorker, Malcolm Gladwell applied epidemiological principles to mass shootings. The article ("How School Shootings Spread") is worth a read.
The theme is contagion -- that a propensity to commit violence is something like the flu, passed from one person to another. In effect, a first shooting makes the unthinkable thinkable to the next marginal, frustrated person who in turn inspires the third, who inspires the fourth and on and on.
Gladwell mentioned Eric Harris, the psychopathic lead actor in the Columbine shootings, whose story influenced disaffected young men who subsequently took up arms in Sandy Hook, Norway, Roseburg and other locations.
Another example is the spate of post office shootings by disgruntled workers in the last century: one in 1975, seven in the 1980s, 10 in the 1990s, and then a break and two more in 2006.
Post office work is not uniquely stressful and people who work in post offices are not uniquely maladjusted. But the first shooting likely set up the second with a malign suggestion: Shooting colleagues is something that angry postal workers do.
Since then, we have adopted the internet, which is not an unmixed blessing. Its comment sections crackle with anonymous contempt and hate. The internet enables the kind of anger that, if expressed in face-to-face situations, would be called out as overreaction or irrationality. It enables galloping id unmodulated by ego or traditional social conventions.
The internet also makes it easy for angry people with limited social control to read about others like themselves -- Eric Harris, the first postal shooter -- who have taken up guns. It lets them think the unthinkable. It inspires at least some of them to do the same thing.
We spend too much time trying to understand the logic of these shooters. They are not rational people. They are unhappy people conditioned to ideas of violence by the actions of other unhappy people. They reason that they can assert control over their unhappiness by shooting innocents.
This is madness. There is no logic to find.