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Tommy Lynn Sells is a 46-year-old man on death row for a single murder, but by his own admission, he killed so many people over the years, he lost count. That level of violent evil makes Sells' brain an object of fascination for neuroscientists hoping to understand what makes a psychopath tick. And as part of tomorrow night's Nightline primetime series, Secrets of Your Mind, Martin Bashir sat down with this unrepentant monster. They were all innocent children, a son, a brother, lives with limitless possibilities ahead of them. But what these boys would grow up to become, brutal killers. Could their brains have provided a roadmap to the sickness that would consume them? First time I killed somebody, it was such a rush. Tommy Lynn Sells was once a little boy growing up in St. Louis, but around the age of 14, he says he became addicted to killing. How many people have you killed? Lord, I don't know. I don't know. 10? Yeah. 20? Probably. 30? It's up there. 50? See, I'm not Billy the Kid and making notches on my holster, so I know it's been a lot. Psychopaths are individuals who lack conscience, they lack remorse, they lack guilt. That's one of the reasons why they terrorise society so much. Sells is on death row for one murder, but he's linked to at least 17 more. The drifter's vagrant lifestyle helped him elude police for nearly 15 years, as victims turned up from coast to coast. I am hatred. When you look at me, you look at hate. When I look at you, I look at hate? 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 and strangling some. Vicious cycle. I couldn't help but be grateful for the glass between us. I don't have no feelings no more, no emotion, no... They showed much poorer... Adrian Raine has studied the brains of people scientifically determined to be psychopaths. One of the key differences that we find in the brains of psychopaths is that there's a structural impairment to part of the brain called the amygdala. This is a part of the brain very much involved in the generation of emotion. It was structurally deformed in the psychopaths. I like to watch the eyes fade, the pupil fade. What do you like about that? It's just like setting their soul free. They just don't have those normal feelings of revulsion and disgust. Sells blames much of his murderous rage on sexual abuse he says he suffered as a child. You also killed children. Some got killed, yes. Now why would that happen? I didn't want them to live through the pain I lived through. I tried to get in this door right here. Two days after his final murder, Tommy Lynn Sells walks police through the crime scene. I opened the window all the way up, looked out like this. And that's how I'm here. We kind of stayed up a little later and we got in a little fight about where we were all going to sleep. We laid there and talked, I bet. We talked about, at the time, Britney Spears and NSYNC. Crystal Sells, ten years old at the time, was sleeping over at her friend Katie Harris's. A decade later, that night is still fresh in her memory. I woke up startled a little bit and out of nowhere and I was kind of confused to a loud noise. I'll walk this girl up. Her friend Katie was asleep on the bottom bunk. I cut her bra. I kind of just like leaned my head up a little bit and I could see that there was this scary old, older man that I'd never seen before. I stabbed her here and then she like jumped back and then I cut her like this right here. He had a hand on her mouth and a knife on her neck and she's looking at me and I just cut her throat and she fell to the ground. She fell down right here. I heard Katie at the bottom of the bed, or on the floor at the bottom of the bunk bed. She was like gasping for air and choking, like she couldn't breathe. Thirteen-year-old Katie Harris lay dying on the floor. He was about to shut off the light and he looked one last time and he noticed that I was there and he didn't hesitate at all. I mean, just shut the door, came right back towards me with the knife. I walked over here and I went like this. The only thing that he said is, move your hands, because I had him up here and he reached over the top bunk and cut my neck. Sells sliced Crystal's neck, severing her windpipe and grazing her carotid artery. I am very sure he thought he killed me. Crystal Sells identified Tommy Lynn Sells as her assailant and her friend's killer, putting an end to his homicidal spree. What did you do to her, sir? Her neck was cut. How did you do that? With a knife. Do you have anything to say to her, the little girl who survived? I don't think she really wants to hear what I get to say. What did you see in his eyes? To be honest, he seemed blank. I mean, there was no emotion. It wasn't hard for him. All he would say is, move your hands. It was just, I mean, after what I'd seen him do, he had no emotion in his eyes. That emptiness evident as we spoke. So what if I called you something that you didn't like, and you'd think about killing me? Well, if we was in a fight and you don't get your head down in the concrete, then, you know, so be it. What happens when my head goes down to the concrete? Well, what do you think happens? It cracks like a coconut. And then what happens? You die. You can see more of Martin Bashir's gripping reports on tomorrow night's Nightline primetime series Secrets of Your Mind here on ABC.