A disgruntled worker shoots up a workplace. A student opens fire at a high school. A pilot crashes a planeload of people into a mountainside. There may never be a convincing explanation for such devastating acts of violence, but experts say certain personality disorders such as extreme narcissism can help push people who want to take their own lives to take those of others at the same time.
But as German prosecutors search for what might have motivated co-pilot Andreas Lubitz to deliberately smash the Germanwings plane carrying 149 other people into the French Alps, many experts caution against speculating on a diagnosis.
"We don't have a clue what was going through his mind," said Dr. Simon Wessely, of the Institute of Psychiatry at King's College London. "Even if we had all of his medical records and had conducted interviews with him, it would probably still be impossible to explain such an inexplicable act."
Ripped-up sick notes from a doctor found at Lubitz's home by German prosecutors suggest the27-year-old had an illness he hid from his employers at Germanwings. Medical documents showed he had an existing illness - which wasn't specified - but no suicide note was found.
A Dusseldorf hospital confirmed Friday that Lubitz had been treated recently, but didn't say for what. Neighbors of Lubitz were shocked at allegations he could have deliberately smashed the plane and said he had seemed thrilled with his job at Germanwings.
They described a man whose physical health was excellent and records show Lubitz took part in several long-distance runs. Germanwings said he had passed all required medical check-ups.
Some experts said it was possible that people who commit such horrific acts of violence might be suffering from mental illnesses like narcissism or psychosis.
Dr. Raj Persaud, a fellow of Britain's Royal College of Psychiatrists, says that in cases of mass murder, people sometimes suffer from personality disorders that make them extremely self-centered.
He and others were speaking generally and had no personal knowledge of the Lubitz case. "People feel that something so terrible has been done to them that this catastrophic act is warranted in exchange," he said. "To them, it feels like the correct balance to equal what they suffered."
Others said that preventing such chilling acts of violence may be nearly impossible if there aren't any obvious warning signs or if the person is able to hide their symptoms. "People can become quite skilled at masking their problems because it's socially undesirable," said Dr. Paul Keedwell, a psychiatrist who specializes in mood disorders at Cardiff University.
Keedwell said it would be unwise to assume Lubitz's deliberate plane crash was an aggressive act. "It's difficult to understand, but what if he was just so wholly preoccupied with ending his own life he didn't have any regard for the other people on the airplane?" he said.
He likened it to people who throw themselves in front of trains without considering the trauma that might inflict on the driver and other passengers. Some experts said mass murders are intended by the killer to do maximum damage, to draw attention to themselves.
"The subject wins fame by doing something the world will remember, even if it's as a negative hero," said Dr. Roland Coutanceau, president of the French League for Mental Health.
He said such acts are sometimes committed by paranoid people angry with their employer or with society at large. "This is a destructive act that (gives) him some kind of immortality," Coutanceau said. "Death is therefore part of his script."