If you were to imagine the type of person who would kill a woman and her young son, torch his own house, and then kidnap the woman’s teenage daughter, you’d probably picture a hulking, tattooed ex-convict or a shaggy-bearded antisocial misfit with a crazy look in his eyes.

Probably the last person you’d picture would be James Lee DiMaggio -- a bland, pleasant-looking 40-year-old telecommunications technician, whose LinkedIn profile touts his expertise at Microsoft Excel and his customer service skills, and whose Facebook page -- since removed -- included inspirational quotes from Albert Einstein, Steve Jobs and poet Maya Angelou.  He was a longtime close friend -- “Uncle Jim”-- of the family he allegedly victimized.

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Puzzlingly, the San Diego area resident -- whom FBI agents killed in a gun battle in the Idaho wilderness last Saturday when they rescued his captive, 16-year-old Hannah Anderson -- seems, at least outwardly, like the antithesis of the deranged predators who inhabit our nightmares.

According to Intelius, an online public records database, DiMaggio had no prior criminal record in California, and he owned the three-bedroom home that he allegedly set afire after killing his two victims.

San Diego County Sheriff Bill Gore admitted to reporters: “When you get a completely irrational act like we've seen here with two murders and a kidnapping, sometimes you might not be able to come up with a rational explanation.”

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But that leads to an even more troubling question. If an apparently normal person -- that is, someone who is functional in everyday living, with no history of violent acts or mental disturbances -- can suddenly morph into an arsonist, killer and kidnapper, what are the chances that any one of us similarly could go off the deep end and erupt into extreme violence?

While there’s little specific research on the subject, crime statistics suggest that the chances of an everyday person suddenly morphing into a brutal killer seem fairly slim. According to a 1997 U.S. Bureau of Justice Statistics study, the average American only has about a 5 percent chance of being sentenced to prison for committing any sort of crime in his or her lifetime.

If a person has managed to stay out of such trouble until his or her 40th birthday -- as DiMaggio did--the rate drops to lower than 1 percent. The same study showed that fewer than 6 percent of those incarcerated for the first time have killed someone.  And the “normal” person who takes more than one life, as DiMaggio allegedly did, must be rarer still.

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A 2003 study by Northeastern University researchers James Alan Fox and Jack Levin found that only 3 to 4 percent of all acts of homicide involve multiple victims.

It’s impossible to rule out that an ordinary person might someday commit extreme, irrational violence. But in most cases, a seemingly normal killer probably isn’t really as normal as he might seem, explains Dr. Alan J. Lipman, a licensed clinical psychologist and founder and director of the Center for the Study of Violence in Washington, D.C.

A sign for then-missing California teenager Hannah Anderson on a fence at El Capitan High School in the Lakeside neighborhood of San Diego County on Aug. 7, 2013. Corbis

While a person may never have killed before, it’s likely that he or she already was afflicted with a hidden genetic predisposition and/or psychological trauma, ticking time bombs that were waiting for the right amount of stress to set them off.

“Typically, with people who haven’t engaged in violent acts previously, you see a family history of violence,” Dr. Lipman says. “And there’s also some dramatic trigger, an event that causes the person to feel extreme hopeless and rage, of the sort that’s expressed in a suicidal and homicidal act.”

Based on the information that has emerged so far about DiMaggio, Dr. Lipman sees him as a textbook example. DiMaggio’s father, a chronic drug abuser with a history of violence, reportedly also became obsessed with a teenage girl in the late 1980s.

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When she rebuffed his advances, the older man invaded her home with a shotgun and handcuffs and attempted to kidnap her. ("I asked him not to kill us, and he said, don't worry you won't feel a thing,” the victim, who now is an adult, told San Diego TV station KFMB.) But the abduction failed, and the elder DiMaggio eventually committed suicide in 1998.

That disturbing family history -- and DiMaggio’s eventual reenactment of his father’s behavior -- lead Dr. Lipman to suspect that the elder DiMaggio may have passed along to the son a genetic tendency for antisocial personality disorder. Individuals with the disorder can seem charming and likable, but that only masks their capacity to engage in impulsive violence with no remorse or regard for others, according to an article on the Mayo Clinic website.

“It’s usually passed from male to male,” Dr. Lipman explains. “It’s likely that the son was carrying that genetic propensity.”

Dr. Lipman also notes that DiMaggio and his siblings were sufficiently worried about inheriting their father’s violent personality that they swore a pact not to emulate his behavior. It was a promise that DiMaggio ultimately failed to keep, when he found himself in danger of losing his home, and began experiencing feelings of attraction toward the teenage girl that he drove to gymnastics meets.

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“When a person has the underlying tendency for the disorder, and then something goes terribly wrong in his life, he experiences feelings of hopelessness and rage that can cause him to become violent and suicidal, even though those things haven’t been expressed before,” Dr. Lipman says. “It’s like being born with a bone that’s got a weak spot, and is just waiting for years for the right pressure to break.”

If there’s any good news, though, it’s that only 0.6 percent of the U.S. population suffers from antisocial personality disorder, according to a 2007 study. So while there isn’t any conclusive research on how many people carry the genetic tendency for the affliction, it’s likely to be only a very small segment of the population, Dr. Lipman believes. That means that you’re unlikely ever to suddenly commit the sort of violence the DiMaggio allegedly perpetrated.

“Without either a history of violence in his or her life in some form, and/or a genetic predisposition to mental illness of this sort, a person would be much less likely -- significantly, though not zero -- to do something like this,” Dr. Lipman says.