Inside the Mind of a Murdering Psychopath

Tommy Lynn Sells claimed he killed dozens of people before he landed on death row.
3:00 | 04/05/14

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Transcript for Inside the Mind of a Murdering Psychopath
Tonight we take you inside the mind of one of the most evil men in America. He's a notorious and admitted serial killer, and the last 48 hours, his story has come to a dramatic conclusion. We got a rare and chilling interview with him for our series, crime and punishment. First time I killed somebody, it was such a rush. Tommy Lynn sells, a man who says he belongs on death row. I was raised to do time. I wasn't raised to live out there in that world. Sells was convicted of killing 13-year-old Katie Harris, stabbing her to death in her bedroom. I like to use a knife. A gun is too violent, too noisy. It just wouldn't fit my hand well. And later, he admitted to killing dozens more. So many, he can't even keep track. I'm not making notches on my holster. I knee it's been a lot. Addicted to killing is the way he characterized it. I don't have an on and off switch. I'm just after that drug. I'm after that feeling. "Nightline" interviewed sells on death row in 2010. One of the rare glimpses into the mind of a serial killer. It's complicated. When people enter my life, they get hurt. The drifter's vagrant lifestyle helped him elude police for nearly 15 years as victims turned up from coast to coast. When you look at me, you know what hate is because I don't know what love is. Two words I don't like to use is love and sorry. Because I'm about hate. His methods for killing were as random as the people he targeted. He raped many before cutting their throats or beating them. Stabbing others. He strangled some. A lot of jerking, a lot of movement. It's a physical -- you become part of it. And described it all with seeming detachment from the horrors H caused. I don't have no feelings no more. No emotion, no -- And no remorse. I liked to watch the eyes paid, the pupil fade. It's just like setting their soul free. We asked sells what would happen if we said something that angered him. If he was in a fight and, you know, smash your head down in the concrete, so be it. But what do you think happens? It cracks like a coconut. How did he become this monster? Sells blames much of his murderous range on sexual abuse he says he suffered as a child. I think every time I killed that guy that molested me all them years. I seen that in my mind over and over. Yet, children were among his victims. I didn't want them to live through the pain I lived through. I tried to get in this door right here. It was Katie Harris' murder that finally ended his killing spree. A few days later, tommy Lynn sells walked police through his crime spree. I lifted the window up and that's how I got in. Kids stayed up a little later and we got a in a little fight about where we were going to sleep. We talked about the time Brittany spears and N sync. Crystal is one of his few survivors. She was sleeping over at Katie's house that night. More than a decade later, it's all still fresh in her memory. I woke upstartled a little bit out of nowhere and I was kind of confused to a lot of noise. I woke this girl up. Her friend Katie was asleep on the bottom bunt bunk. I cut her bra. I could see there was a scary older man that I have never seen before. I stabbed her here and she jumped back and I cut her like this right here. He had a hand on her mouth and on her neck. She was looking at me, he cut her throat and fell to the ground. 13-year-old Katie Harris lay dying on the floor. He was about to shut off the light, he noticed one more time that I was there, he just shut the door and came back towards me with the knife. The only thing he said was move your hands. He reached over the top of me and cut my neck. She severed her windpipe. I am very sure he thought he killed me. Crystal identified tommy Lynn sells as the assailant. To be honest, he seemed blank. I mean, there was no emotion. They lack conscious, they lack remorse, they lack guilt. That's one of the reasons why they terrorize society so much. Experts say sells offers a textbook example into the mind of a serial killer. He's been studying how and why the adorable little boy pictured here turned into brutal killers. Could it have been the makeup of their brains? Biology coming together with environmental insults which raised the odds of an individual becoming a violent criminal offender. Recent researchers sulg that psychopath Thi may actually be evident in the brain and that a simple brain scan could reveal key differences in the minds of serial killers. The advent of molecular gin nettics, we've been able to peer into the minds of murderers for the first time. But for some of the families of his victims, ending that brain function was important. Sells was on death row for more than 13 year, the most recent delay, over the exact makeup of the drug cocktail that would be used to kill him. Sells said the pain of the death plenty paled in comparison to his life. I suffer more here than I'll ever suffer in that grave. When they kill me, I'll be free then. Last night, he was executed at 6:27 P.M. When asked if he wanted to make a last statement, he simply answered no. For the family of Katie Harris, it was long overdue. Now it's a day of closer and celebration. Justice has been served.

This transcript has been automatically generated and may not be 100% accurate.

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