"Every mean, vicious psychopath once upon a time, was an innocent young baby," said Dr. Adrian Raine, a neurocriminologist at the University of Pennsylvania
"It's only recently, with the advent of molecular genetics and brain imaging techniques that we have been able to peer into the minds of murderers really for the first time."
Tommy Lynn Sells is an extreme example of someone with a murderous mind. As a young boy growing up in St. Louis, Sells was addicted to killing by the time he was 14. He estimated that he has killed dozens of people.
"I am hatred. When you look at me, you look at hate," he said. "I don't know what love is. Two words I don't like to use is 'love' and 'sorry,' because I'm about hate."
The 45-year-old is on death row in Texas for one murder, but he's been linked to at least 17 others. His drifter lifestyle helped him elude police for nearly 15 years as his victims turned up from coast to coast. Sells said his drug use fueled his killing sprees.
"The first time I did a shot of dope, it was the best feeling I ever had in my life. The first time I killed somebody, it was such a rush," he explained. "It was just like that, a shot of dope every time I did it, it was that rush again, and I started chasing that high."
Sells appeared very nonchalant when asked about the victims he had slain, and spoke very matter-of-factly when recounting his killing methods: beatings, stabbings, strangling -- he even raped many of his targets before slicing their throats.
"I like to watch the eyes fade, the pupil fade. It's just like setting their soul free." Sells said without showing emotion. "I don't have an on-and-off switch. I'm just after that drug. I'm after that feeling."
One of the more horrific crimes Sells has confessed to committing was the slaying of 30-year-old Eileen Dardeen and her family.
Dardeen was seven-and-a-half months pregnant when Sells allegedly beat her to death. During the attack, she went into premature labor and gave birth to a baby girl.
Investigators determined the newborn was alive when she was born, but was later found bludgeoned to death, as was Dardeen's three-year-old son.
Her husband's body turned up a few days later. He had been shot three times in the head.
Investigators involved with the Dardeen case said if Sells were not already sitting on death row in Texas, they would have arrested him for this crime.
Sells blamed his murderous rage on having an abusive childhood.
"I didn't want them to live through the pain I lived through," Sells said in explaining why some of his victims had been children.
Raine offers reasons why someone like Sells could be so off-the-cuff about such grisly crimes.
"Psychopaths are individuals who lack conscience. They lack remorse, they lack guilt," he said. "That's one of the reasons why they terrorize society so much."
Through his research on the brains of people who have been scientifically determined to be psychopaths, Raine uncovered something faulty with their wiring.
"One of the key differences that we find in the brains of psychopaths is that there is a structural impairment to the part of the brain called the amygdala," he explained. "This is the part of the brain very much involved in the generation of emotions. It was structurally deformed in the psychopaths."
But he also acknowledged that a rough childhood can also be a strong contributing factor to violent behavior.
"It's biology plus environment," Raine said. "It's biology coming together with environmental insults which raise the odds of an individual becoming a violent criminal offender."
Sells has left only one known survivor during his terrifying rampage.
Krystal Surles was just 10-years-old when Sells tried to murder her while she was sleeping over at a friend's house in Del Rio, Texas.
Asleep on the top bunk, Surles awoke in the middle of the night to Sells killing her friend, 13-year-old Katy Harris.
Though it happened a decade ago, it's a night that remains burned in her memory.
"He had a hand on her mouth and the knife on her neck, and she's looking at me, at the bunk bed," Surles recalled. "He just cut her throat, and she fell to the ground."
Frozen with fear, Surles said Sells then walked towards the door, about to shut off the light, when he looked around the room one last time.
"That's the first time he noticed me," she said. "He ... didn't hesitate at all. I mean, just shut the door, came right back towards me with the knife."
"The only thing that he said is, 'move your hands' ... and he reached over the top bunk and cut my ... cut my neck."
Sells severed Surles's windpipe and grazed her ceratoid artery. She played dead on the floor in a pool of blood until she thought Sells had gone, then she ran for help at a neighbor's house.
Surles later identified Sells as her attacker and as the man who had brutally killed her friend, which finally put an end to his killing spree. He was later found guilty.
Sells said he still remembers the one that got away. "There's not a day goes by that I don't think of her," he said.