He was described, in the immediate aftermath of the Germanwings crash, as a cheerful and careful pilot, a young man who had dreamed of flying since boyhood.

But in the days since, it has seemed increasingly clear that Andreas Lubitz, 27, the plane’s co-pilot, was something far more sinister: the perpetrator of one of the worst mass murder-suicides in history.

If what researchers have learned about such crimes is any indication, this notoriety may have been just what Mr. Lubitz wanted.

The actions now attributed to Mr. Lubitz — taking 149 unsuspecting people with him to a horrifying death — seem in some ways unfathomable, and his full motives may never be fully understood. But studies over the last decades have begun to piece together characteristics that many who carry out such violence seem to share, among them a towering narcissism, a strong sense of grievance and a desire for infamy.

Adam Lankford, an associate professor of criminal justice at the University of Alabama, said that in his research on mass killers who also took their own lives, he has found “a significant number of cases where they mention a desire for fame, glory or attention as a motive.”

Before Adam Lanza, 20, the Sandy Hook Elementary School shooter, killed 20 children, six adults and himself in 2012, he wrote in an online forum, “Just look at how many fans you can find for all different types of mass murderers.”

Robert Hawkins, 19, who committed suicide after killing eight people at a shopping mall in Omaha in 2007, left a note saying “I’m gonna be famous,” punctuating the sentence with an expletive.

And Dylan Klebold, 17, of Columbine High School fame, bragged that the goal was to cause “the most deaths in U.S. history…we’re hoping. We’re hoping.”

“Directors will be fighting over this story,” Mr. Klebold said in a video made before the massacre.

If authorities know what might have driven Mr. Lubitz, they have not made it public. Prosecutors said last week that it was now clear that he planned the crash, researching ways to commit suicide and how to operate the cockpit door on his iPad.

Image Andreas Lubitz in 2009. Credit... Michael Mueller

Lufthansa, Germanwings’s parent airline, has said that Mr. Lubitz had reported suffering in the past from severe depression, and prosecutors have said he had talked to a counselor about suicide.

Yet mental health experts who study mass murder-suicides said that depression and thoughts of suicide, which are commonplace, fall far short of explaining such drastic and statistically rare acts.

“People want an easily graspable handle to help understand this, to blame something or scapegoat,” said Dr. James L. Knoll, the director of forensic psychiatry at the State University of New York Upstate Medical University.

But to zero in on depression is “a low-yield dead end,” he said, adding, “There’s something fundamentally different here, aside and apart from the depression, and that’s where we need to look.”

Serious mental illness, studies of mass killers suggest, is a prime driver in a minority of cases — about 20 percent, according to estimates by several experts. Far more common are distortions of personality — excesses of rage, paranoia, grandiosity, thirst for vengeance or pathological narcissism and callousness.

“The typical personality attribute in mass murderers is one of paranoid traits plus massive disgruntlement,” said Dr. Michael Stone, a forensic psychiatrist in New York who recently completed a study of 228 mass killers, many of whom also killed themselves.

“They want to die, but to bring many others down with them, whether co-workers, bosses, family members or just plain folk who are in the vicinity.”

Mr. Lubitz, Dr. Stone noted, now ranks among the deadliest of mass killers, in a league with Adilson Marcelino Alves, who in 1961 killed as many as 500 people in a circus fire in Brazil, or Timothy McVeigh, the Oklahoma City bomber, who killed 168 people and injured more than 680 others.

Dr. J. Reid Meloy, a forensic psychologist who consults on threat assessment for universities and corporations, said perhaps the most salient feature of mass killers was their belief that they had been wronged.