While some of these incidents from a perpetrator's past are indeed crazy, they have poor positive predictive value: the most recent shooter apparently punched a woman over a booster seat at an IHOP. But such ironic and strange incidents happen all the time, which I know as an avid reader of News of the Weird. Attempting to predict who will go bezerk is like looking for a needle in a haystack: many people do minor strange, crazy things, and even do so multiple times, but most people who do minor strange things will not commit homicide. Even looking at more severe incidents, such as shooting a brother at age 20, may not necessarily predict future violence. Though allegedly sending a bomb to a dissertation committee member seems more likely to predict future violence, she was cleared of that charge.
Attempting to predict who will snap begs the question. We can't. Even if we had Big Brother comprehensive databases of every person's past, and were willing to disregard rules of evidence and dropped charges, we couldn't predict severe violence. The real issue is not which people are likely to snap, but rather why our gun control policy allows people such easy access to firearms that when they do snap they can do so much damage. Human psychology is fallible, but in countries without easy access to firearms, the damage comes in the form of broken objects, bruises and broken bones, and perhaps even a stabbing. Firearms create more damage, and damage that is most likely to be deadly.
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Speaking of News of the Weird, here's one from last year about a Yale PhD and professor:
Love Can Mess You Up: Before Arthur David Horn met his future bride Lynette (a "metaphysical healer") in 1988, he was a tenured professor at Colorado State, with a Ph.D. in anthropology from Yale, teaching a mainstream course in human evolution. With Lynette's guidance (after a revelatory week with her in California's Trinity Mountains, searching for Bigfoot), Horn evolved, himself, resigning from Colorado State and seeking to remedy his inadequate Ivy League education. At a conference in Denver in September, Horn said he now realizes that humans come from an alien race of shape-shifting reptilians that continue to control civilization through the secretive leaders known as the Illuminati. Other panelists in Denver included enthusiasts describing their own experiences with various alien races. [Rocky Mountain Collegian, 9-28-09]