Notorious serial killer Dennis Rader, known as the “BTK Killer,” revealed what drove him to kill 10 people in a documentary which aired on the Oxygen network on September 2.

Rader, 73, talked about his upbringing in “Snapped: Notorious BTK Serial Killer,” a two-hour documentary which explored how—and why—a seemingly normal family man carried out heinous murders over a 17-year period in Wichita, Kansas.

In a never-before-seen interview shown in the documentary, Rader told KAKE-TV reporter Larry Hatteberg in 2005 that a “demon” inside of him caused him to kill 10 people, which included two children. He claimed the demon came into his life when he was a child.

“I mean, I have a lot of feelings for them,” he said of his victims. “I guess it’s more of an achievement for this object in the hunt. Or sort of more of a high, I guess.”

Rader went on to recount an instance when his mother got her ring caught in a couch spring and desperately sought help to free her hand. He recalled feeling a sense of excitement as his mother looked to him in “terror.”

Rader strangled the majority of his victims, using pantyhose, rope, and even his bare hands. Giving himself the name “BTK,” an acronym for “bind,” “torture,” “kill,” Rader relentlessly taunted police until his arrest in 2005. The Mail noted that Rader corresponded with Wichita police using a floppy disk that they ultimately traced back to his church.

According to Fox News, the ex-church president and code inspector pleaded guilty to the 10 murders, telling the court that he committed the slayings for his sexual gratification.

“How could a guy like me, church member, raised a family, go out and do those sort of things?” Rader mused during the documentary.

“I want the people of Sedgwick County, the United States and the world to know that I am a serial killer…It’s a dark side of me.”

[Featured image: Dennis Rader/El Dorado Correctional Facility]