Transcript for What else Dennis Rader, the BTK serial killer, was hiding from his family: Part 7
?????? I'm Dr. Katherine ramsland. My area of expertise is primarily serial killers. When Dennis Rader was arrested in 2005, I saw this as a very unique opportunity for me to use what I knew about serial killers to structure a book by a serial killer. I wrote letters to him. He wrote multiple letters, long letters, back to me. We talked on the phone every single week. I visit him in the prison. Dennis Rader was arrested on Friday in Wichita and he has confessed. He knew he was caught. There was no question. We had not only DNA, but we had all the evidence we were getting at his house. He told us that we would discover what he described and what he termed his mother load. Information that covered from the Otero murders all the way through Dolores Davis. There were journals and drawings. He had materials that he collected from each of his victims. Clothing, car keys. He had compartments everywhere. He had false bottoms in a closet, he had stuff in a crawl space, he had stuff in a tree house he had made for his kids. Also there were polaroids of himself in his autoerotic activities, that the police initially thought were male victims that they didn't know about. What we would discover is photographs of himself where he would be dressed up as a female. Sometimes he would have a mask on and a wig and he would be tied up. He was dressing himself up in bras and underwear, torturing himself, hanging from trees, taking photographs of himself half buried with a polaroid camera that he'd hook up to this foot and pull the trigger. He took thousands of pictures with that polaroid. He took the photos of himself to relive the experiences. That was part of what he needed to be aroused. Reporter: What did you make of all that? Horrified. To have somebody that seemed so prim and proper, to find out that was his other side, that was just weird. Like, remove all the crimes, just that is, like, shocking. Serial killers have a pattern. Usually they're abused as children. He flatly denied any physical abuse, sexual abuse. All of the hallmarks of a person that turns into a serial killer, he claimed he didn't have. He always had a very, very healthy fantasy life. Rader had at a very young age connected sexual excitement with violence. In November 1959, about 200 miles west of Wichita, in a town called Holcomb, Kansas, a family of four was murdered, tied with ropes first. Dennis Rader, at the age of 14, was in a car with a girl he had a crush on. He heard this on the radio, this alarming story of the clutter family being murdered in their homes. He immediately wanted to kill the girl that was in the car. The in cold blood case in Kansas is so shocking, they make a movie out of it. Make one move, holler once and we'll cut their throats. To Dennis, it really taps into his fantasy and obsessions. His reaction was not horror as most people's were, it was arousal, attraction. He wanted to do that too. Were there things in Katherine ramsland's book that angered you? Yeah. The book was very personally difficult to read. Rader said that his wife one time came home unexpectedly and witnessed him in her slip, hanging himself or at least preparing to do so. He tried to explain it. He says she went to a counselor and was reassured this is a man thing and it wasn't necessarily dangerous or terrible, but she wanted him to keep it out of the house. Just don't do it in front of me. I called her. I said, look, he's claiming this thing in this book that's about to come out, and I'm like, did that happen? She's like, no, no, no, no. She said, like, no, like, five or six times. So then it becomes this issue who are you going to believe. Are you going to believe the sane Normal woman which is my mother? Are you gonna believe the narcissistic psychopath that you know lies all the time? Rader does have a lot of psychopathic behaviors and we know from brain research that they have very shallow emotional roots. So he doesn't have a conscience about lying or deceiving or manipulating or pretending to be somebody he's not. When you try to sort this out, is your dad mentally ill or is he evil? He's definitely mentally ill. But he'snot insane. He very much knows what he's doing and what he did. He's just a sick, sadistic murderer, is what it comes down to, not some fascinating criminal. I think to dismiss Dennis Rader or any other serial killer or mass murderer as a one dimensional being, is to make yourself unsafe because you will not spot the real monsters if you think they're so easy to see. Early on, when my father wrote me in 2005. He said, like, he's so sorry things got away from him and, like, cat and mouse caught up with him. You will always be my baby girl I raised right, proud, independent. Hopefully someday your heart will mend and you can forgive me. Life before the arrest was a good time and the dark side took me away. Reporter: How could you encore respond with him? I mean, people would wonder like, why wouldn't you just cut him off? I wasn't corresponding with btk. I'm never corresponding with btk. I'm talking to my father. I'm talking to the man that I lived with and loved for 26 years. I still love my dad today. Reporter: You still love him today? Because I love the man that I knew. And so, you know, clinically there's, like, criminologists or psychologists that would say, your father's a psychopath and he's incapable of feeling. But I don't know -- I don't know a psychopath. That's not the man I knew and loved. So, like, I -- I have a tendency to want to compartmentalize and disassociate and say, over here for 26 years, this man that I adored and loved that could sometimes be gruff and a couple times was abusive, and that's not okay. You know, and then over here is this insane, torturous, violent, horrific man. I don't know that man. So if I'm going to get up every morning and live my life, I better come to learn how to get back here.
This transcript has been automatically generated and may not be 100% accurate.