The Mystery of Mass Killers

hard times and give them a reality check.”

“They have a history of frustration,” he went on. “They externalize blame. Nothing is ever their fault. They blame other people even if other people aren’t to blame. They see themselves as good guys mistreated by others.”

Jeffrey Swanson, a professor of psychiatry and behavioral sciences at Duke University in Durham, North Carolina, said these individuals often feel they do not belong, yet frequently live in “smaller town settings where belonging really matters.”

Mr. Harper-Mercer showed signs of such isolation and despair. Like others, he appeared smitten by past mass killers. “They see them as heroes,” Dr. Fox said. “Someone who wins one for the little guy.”

Elliot O. Rodger, a 22-year- old California college student, had not had any friends since grade school. What little interactions he had seemed to be online, while playing the video game World of Warcraft. Many mass killers gravitate to violent video games, as do many young men in general, though this could be more a symptom of their isolation than a cause of their violence.

A parent of an elementary school classmate said her husband had refused to allow their son to spend the night with Mr. Rodger, who would hide in their home when he would visit. Simon Astaire, who served as the family spokesman, said at the time, “He was as withdrawn as any person I ever met in my life.”

As a teenager, he received a diagnosis of a developmental disorder.

At Santa Barbara City College, Mr. Rodger clashed with his roommates and lived a life online. He stopped attending classes, and he posted videos about being rejected by women. Not long before he acted, he posted a video to YouTube. It showed him sitting behind the steering wheel of his BMW, ranting about his isolation, the women who had shown no interest in him and his disappointment at being a virgin. He complained, as well, about all the sexually active men who were enjoying life.

On May 23, 2014, he stabbed three men to death in his apartment, then drove off and shot three others from his car. After two shootouts with sheriff’s deputies, he killed himself.

But sometimes the reasons may be clear only to them. Nearly a year ago, Jaylen Ray Fryberg, a popular 14-year- old football player outside Seattle, texted two cousins and three friends to meet him in the cafeteria, then opened fire on them before killing himself. Four died. He had posted cryptic messages on social media: “It breaks me. ... It actually does. ... I know it seems like I’m sweating it off. ... But I’m not.”

Other mass killers strike against broad categories — a religious group or immigrants or women.

Dylann Roof, the 21-year- old white high school dropout charged in the June massacre of nine black people at Emanuel A. M. E. Church in Charleston, had a website where he posted a fourpage screed about his quest for white supremacy.

The least common variation is the indiscriminate public killing: when people die because they happened to be where the killer was.

Kurt Myers, 64, was barely scraping by in the upstate New York village of Mohawk. He was 64, had not had a job since 2006 and was more than $21,000 in debt. But he had no known history of mental illness or of interactions with law enforcement, except for a 1973 arrest for drunken driving. On the morning of March 13, 2013, Mr. Myers is believed to have set his apartment on fire. He picked up a shotgun. He drove to a nearby barbershop, where he shot four people, killing two, then went to a carwash and lube place and murdered two more. He holed up in an abandoned bar, where he was killed by police the next day.

Mr. Myers had not had much to do beforehand with the people killed or places where he killed them.

Dr. Fox believes that, in the case of family massacres, the perpetrators usually “are not seriously mentally ill, but vengeful.” But “for the purely random attackers, that’s where you find psychotic thinking.”

Dr. Duwe, among his 160 cases of mass public killers, concluded that 61 percent had a serious mental health disorder. Paranoid schizophrenia was the most common ailment, followed by depression.

In September 2013, Aaron Alexis, 34, a former Navy reservist who worked for an information technology company, planted himself above an atrium at the Washington Navy Yard and fired on everyone he saw, killing 12 of them. He was shot and killed by the police. A month before the shootings, Mr. Alexis got into an argument with a family at an airport in Virginia. Glynda Boyd recalled how Mr. Alexis had asked her, “Why is she laughing at me?” He was referring to her 78-year- old aunt, who was in a wheelchair.

Dr. Swanson said only 7 percent of people with mental illnesses might do anything violent in a year, “something as minor as pushing.”

With many of the killers, the signs are of anger and disappointment and solitude.

“You can’t go out and round up all the alienated angry young men,” Dr. Swanson said.