"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."
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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."