(ABC News) -- Tom V. Odle was only 18 when he stabbed his father to death with a butcher knife in an LSD-fueled rage. Then, he waited anxiously for his mother to return to their Mount Vernon, Ill., home and when she did, he repeatedly stabbed her in the neck until she was dead.
The murder of his two brothers and his sister were to follow -- one strangled, the others stabbed.
The family mass murder in 1985 shocked the nation and Odle was sentenced to death by lethal injection. At the time, he was one of only eight people in the U.S. to be sent to death row for family mass murder since 1870.
But when fears of wrongful convictions swept through Illinois, a moratorium on executions in 2001 changed Odle's fate. He suddenly faced life in prison instead of death, and decided to atone for his crimes.
In 2003, after 17 years on death row, he wrote a letter to neuropsychologist Robert Hanlon of Northwestern University Feinberg School of Medicine, trying to understand why he had committed such a heinous act and how he might help prevent other such tragedies.
"I'd had no contact with him for several years," Hanlon told ABCNews.com. "Then out of the blue, I received a letter essentially asking me questions to clarify aspects of his antisocial personality disorder, which I had diagnosed him with, and asking me why I thought he committed the crime he did."
Now 47, and an inmate at the Dixon Correctional Center, Odle shares a credit in a new book by Hanlon, "Survived By One: The Life and Mind of a Family Mass Murderer," a collaboration based on letters and therapeutic interviews with Odle to recall his struggles through childhood abuse.
"His motivation was revenge, directed primarily toward the mother for the abuse she had enacted and against his father for allowing the mother to abuse him for so many years," -- Robert Hanlon, neuropsycholgist
"It may be difficult for a lot of us to appreciate, but this guy who is on death row and anticipating he's eventually going to be executed," Hanlon said. "Suddenly that changed, and out of nowhere, he realized he if lived another 40 years, he wanted to understand what he did and what mistakes he made."
Hanlon said Odle was highly intelligent and "intuitive" and his mental facilities were fully intact. He also did not receive compensation for his contributions to the book.
"To me, it seemed like the appropriate thing to do," Hanlon said. "A lot of the information provided in the book came from the letters he wrote me in the process of self-exploration and analysis."
Hanlon said Odle felt "extreme regret and guilt."
"Beyond that, he feels shame not only for what he did with is life and the murders of his family, but how it affected his community and other relatives, friends of the family and teachers," he said. "Everyone knew his family and the monumental crime."
Unlike other modern family mass murders, money had "nothing to do with this crime," Hanlon said.
In another high-profile family case in 1989, Erik and Lyle Menendez, 25 and 28, waited for their wealthy parents in their Beverly Hills mansion, then blasted them with shotguns as they watched television.
"[Odle's] motivation was revenge, directed primarily toward the mother for the abuse she had enacted and against his father for allowing the mother to abuse him for so many years," he said. "There were also personality factors that Tom Odle possessed at the time, compounded by drug abuse that increased his impulsivity and aggressive tendencies."
Odle killed his parents, Carolyn and Robert Odle, both 39, and his three siblings, Scott, 10; Sean, 13; and Robyn, 14
His mother was physically and emotionally abusive -- mostly targeting young Tom and his younger brother Sean -- and his father passively allowed the abuse to take place, according to Hanlon.
Sean was briefly removed from the home after he told school officials he was often denied food and that at times their mother had chained the boys to their beds while she went out at night. But he was later returned home after the Odles "begrudgingly" took a parenting course required by the Illinois Department of Children and Family Services.
Diagnosed as "rebellious, oppositional, defiant, manipulative," Odle became involved in neighborhood theft, excessive drug use -- mostly cocaine and marijuana -- and experienced depression and complicated relationships with women, friends and family.
By November 1985, his parents no longer wanted him to live at home and the day of the murders was his deadline for moving out.
The key question in the crime is why Odle killed his siblings.
"He was in a homicidal state of mind," Hanlon said. "There were two bodies -- both of his parents were laying dead on the floor. It was kind of like a freight train out of control -- kind of a nihilistic drug-fueled termination of the family unit."
Odle had no major disabling illness like schizophrenia or bipolar disorder, though he was significantly depressed at the time.
"He was not insane," Hanlon said. "He could understand and appreciate, despite the drugs he was using, the criminality of his conduct."
Previous research by Hanlon compares premeditated and well-organized executions, usually for financial gain, versus the Odle case, which was "an impulsive act."
According to previous research by Hanlon, those who commit impulsive acts tend to be mentally impaired or have psychopathic traits like lack of remorse or empathy.
"Tom Odle doesn't fit the study," he said. "He was quite unusual, which to me made this a very interesting case and worth exploring in depth."
Hanlon is not the first to get up close and personal with a killer. Truman Capote notoriously courted two young men, though not related, who wiped out a Kansas family and wrote about it in his 1960 book, "In Cold Blood."
Mary Papenfuss, who wrote the 2013 book, "Killer Dads: The Twisted Drives That Compel Fathers to Murder Their Own Children," analyzed five types of "family annihilators" and their motivators.
One, Peter James Wilson, who killed his 5-year-old step-daughter Clare Shelswell while on vacation in Hoodsport, Wash., in 2010, wrote a chapter in her book. He was sentenced to 55 years in prison.
"He felt rage building up in the midst of an argument with his wife," Papenfuss said. "I think he had a really difficult time articulating, like he went into another state. He felt if the little girl wasn't there, he and his wife wouldn't fight. ... He cut her throat and she bled to death slowly."
Wilson said he had come from a broken home and was "shuttled" between his mother and father.
"He recognized early on he had sort of impulse-violence issues," she said. "He told me at one point an angry brother had pushed him down the stairs."
Before he killed his step-daughter, the man said he had felt "disrespected" by his wife, according to Papenfuss.
"I wrote my book searching for answers but also to raise awareness," she said. "We have to care more about this and pay attention to it."
Like Papenfuss, Hanlon said his hope for the collaboration with Odle was to understand the "behaviors, dynamics and relationships" to help identify potential family mass murders and to intervene.
Both suggest the biggest factor in the shaping of a potential family killer is a history of childhood abuse.
"It's hard not to overstress that," Hanlon said. "It doesn't matter how long or how much time has passed, those experiences are rarely ever forgotten or forgiven and stay with people and influence their character and their choices in relationships.
"Often a precipitating event occurs even when that deck is stacked with factors -- a sudden loss that occurs in a person's life, something has been taken away from them," he said. "In this case, in his view his parents were abandoning him and ejecting him from the home and it came on the heels of his girlfriend basically rejecting him."
Police found the massacre when Odle's father didn't show up for shift work. They immediately did a search for Odle, who was staying at a local hotel. He never denied murdering his family.
But what disturbed Hanlon most about Odle's story was the parental abuse.
"She and her husband had managed to conceal that abuse quite effectively for many years and it wasn't exposed," he said. "Certainly, if people had been more aware and knew what to look for and intervened, they probably would be alive today."