Vester Flanagan was determined to explain himself. After murdering two TV journalists (and former coworkers) during a live broadcast in Virginia on Wednesday, he faxed three different suicide notes to ABC News that were strewn with absurdities, contradictions, and baffling rationalizations. Flanagan, who was black and went by the name Bryce Williams on air, complained about racism against him at the TV station, WDBJ, but then confessed to acting "somewhat racist against whites, blacks and Latinos" himself. He admitted to killing his own cats—because he was fired unfairly. He said how much he loved them, then gave a long, sadistic account of the execution; he killed his favorite first, and she didn't go quickly. Then he described the extreme trauma on the second cat's face as he killed her. He called it a gruesome scene—then decided he owed them a decent burial. Through the rambling diatribe, victimization is a constant theme: "Haters" hounding, demeaning, and harassing him at every turn. "I have a right to be outraged!!!”

Before my research into the Columbine High School massacre, I would have been flummoxed. But after 16 years studying these types of killers, his inconsistencies couldn't have been more consistent. Flanagan was a classic "injustice collector."

Retired FBI profiler Mary Ellen O'Toole, who is widely regarded as one of the smartest people alive on these cases, described Flanagan as such on an appearance on CNN last week. And it was an easy call—Flanagan could be the case study. O'Toole published a primer on injustice collectors in the journal Violence and Gender last year, and many of her descriptors could be lifted right out of this case: "nurses resentment ... accumulating real or imagined slights, insults or putdowns ... could not get along with his co-workers ... disproportionate and aggressive response." Collectors magnify petty "injustices" and perceive them as intentional and purposeful. Over time, he forms a worldview of himself as victimized, bullied, discriminated and disrespected.

The angriest collectors lash out in erratic and disproportionate ways. But they almost never kill out of the blue.

How does this lead to murder? The angriest collectors lash out in erratic and disproportionate ways. But they almost never kill out of the blue. One of the most notorious killings of this type was the Bath, Michigan massacre in 1927. Long before the big crime, Andrew Kehoe left a searing trail of fierce retaliation for petty offenses. He killed a family friend's dog, telling her it barked too much. He beat his own horse to death for laziness. He fought with co-workers, lost his job, lost re-election and was losing his house and farm. Then he set about his revenge. He spent months wiring the school basement with hundreds of pounds of dynamite. On May 18, he murdered his wife, who had been burdening him with tuberculosis treatment, and firebombed his farm, burning two trapped horses to death. Then he detonated the school. Forty-five people died, including 38 kids. At least 58 were injured. Though widely forgotten, because it did not breed copycats, it remains the worst school massacre in American history.

After Columbine, O'Toole led a comprehensive FBI investigation of school shootings, which included an in-depth review of eighteen previous cases. She authored its 2000 report. In quite a few cases, the pattern of injustice collectors leapt out at her team. These very angry collectors responded to "injustice" with revenge fantasies—but not the way most people think of revenge, or wield it. For most of us, revenge is highly personal, and tit-for-tat: He did this to me, I'll pay him back with that. Collectors who kill get overwhelmed by their enemies list. Everyone is on it. Chance encounters, people he doesn't even know: That bus driver who braked too abruptly three summers ago; the woman at Starbucks who coughed too much in 1983; the dirty look from the girl in high school Spanish class, sophomore year. They ruminate over petty slights, often imaginary, and spit them back in specific detail years later.