What could lead someone to kill not only themselves but 149 innocent strangers, too?
Psychologists say homicidal-suicide is vastly different to suicide and usually involves deep-seated resentment or a burning desire for revenge.
French prosecutors revealed on Thursday that Germanwings co-pilot Andreas Lubitz probably steered Flight 4U9525 into the French Alps deliberately, after locking the pilot out of the cockpit.
Those who commit murder-suicide have more in common with a suicidal person than a murderer, said Professor James Ogloff, a psychologist and the director of Swinburne University of Technology's Centre for Forensic Behavioural Science.
"Rather than being aggressive and outwardly hostile, we actually see they're quite depressed and have their own psychological issues that are haunting them," he said.
However, if the bombshell revelations prove true, Lubitz's horrific act has one crucial difference to a lone suicide, he said.
"The difference is there's a total disregard for the fact other people are going to be affected, or there is a degree of resentment that's driving it.
"It's usually very unlikely that a suicidal person wants to take out others, especially strangers. In these sorts of cases, people are either particularly resentful and angry against society broadly, or against a particular organisation."
Police have searched Lubitz's home as they try to piece together his life and understand why he may have calmly and silently steered the plane into a mountain.
He reportedly suffered a severe depressive episode six years ago but had an affluent upbringing, a seemingly normal adult life and had recently been cleared to fly.
A report in German tabloid newspaper Bild Zeitung quoted an anonymous security source saying Lubitz was going through a deep crisis due to a relationship breakdown with his girlfriend.
Clinical psychologist Fiona Shand, a researcher at the Black Dog Institute in Sydney, said it was likely the world would never know why Lubitz took so many strangers down with him, if indeed he did crash deliberately.
Research and understanding of murder-suicide is very limited because, by the very nature of the act, those involved don't usually survive to give valuable insights.
However, Dr Shand said murder-suicide usually involved a degree of psychopathy or psychosis.
"Suicide, at least in Australia, tends to be driven by people who are feeling as if they're a burden on people around them and wanting to relieve that burden on loved ones," she said.
"That's a crucial difference. The majority of suicides occur without harming anyone else."
She said suicide was often referred to as an "internalising" disorder but murder-suicide would be on the "externalising" spectrum.
Professor Ogloff said even psychological experts had great difficulty in understanding murder-suicide.
In high functioning individuals such as Lubitz, it can be nearly impossible to detect.
"The safest thing to say is that it is incredibly rare and you need to be quite vigilant to people having outward indicators of distress at a personal level," he said.
"In a position such as a pilot, you'd want to be incredibly conservative about their capacity to continue work."
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