'I Am Hatred': What Turns Someone Into a Psycho-Killer?

"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."

Krystal Surles's Survival

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.

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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.

Watch the full story on Tommy Lynn Sells and his victims, as well as other profiles of psychopathic killers tonight on "Secrets of Your Mind" at 10 p.m. ET