Gilgo Beach serial killer Rex Heuermann strangled his first victim to death inside his Dodge Ram pickup truck in November 1993 — a grisly murder he later claimed wasn’t planned and “just happened.”

That changed over the next 17 years, as the Massapequa Park dad developed a meticulous four-day killing system to torture and strangle seven other sex workers in his basement “kill room” while his family was away, according to the final chapter of “The Gilgo Beach Killer: House of Secrets.”

Titled “The Confession,” the latest installment of the Peacock series in partnership with The Post reveals chilling new details of the serial killers decades-long reign of terror through jailhouse confessions to his family and a psychotherapist prior to his shocking guilty plea in a Suffolk County courtroom.

“He said that his demons got to him,” his daughter, Victoria Heuermann, told the producers. “When he was in a certain opportunity or there was a certain catalyst in front of him, that would start to create these dark urges. There was a sickness.

“I asked him, ‘Did you see them as somebody’s daughter?’” she added. “He told me he didn’t even see them as human.”

The bombshell new details come in the aftermath of Heuermann’s April 8 guilty plea where he admitted to killing eight sex workers by strangulation and dumping them along desolate stretches of Long Island between 1993 and 2010. Several of the victims were dismembered in the family basement, according to the revelations.

Central to the episode is veteran psychotherapist Alison Winter, who was brought into the case by Robert Macedonio, the attorney for Heuermann’s ex-wife, Asa Ellerup, to counsel the tormented family.

Winter’s involvement included disturbing jailhouse interviews with the notorious killer.

“Rex, in later high school, shares with me [that] he starts having thoughts that he knew they were unhealthy,” Winter said. “He did not know what they were per se. He did not know how violent they were. But he knew his thinking wasn’t right.

“So, he started pornography, books on death. He became very fascinated with dissecting a human body,” she added. “And the sex and the pornography and the human body and all of that, in a mind that’s not healthy is a very dangerous recipe.”

Winter also sat down with John Douglas, a retired FBI agent and profiler whose book “Mind Hunter,” is the textbook on the serial criminal mind — and who offered a haunting analysis of Heuermann.

“It’s the violence that really turns him on,” Douglas said. “He put these victims through hell. It wasn’t a swift kill. It didn’t have to be a swift kill because no one was home. No one could hear the screams and yells coming from his cellar.

“He’s definitely a psychopath,” he said. “I think he has a lot of hidden secrets here. He’s a malignant, narcissistic, sadistic psychopathic serial killer. Had he not be apprehended I think he would kill more.”

Douglas also suggested that there might be even more Heuermann victims.

“I don’t believe he started killing at 30,” the former agent said. “There may be cases that he does not want to admit to because in those states, like where he has a house, I believe, in South Carolina, they got the death penalty and they use it.”

Police in the Carolinas, where Heuermann and his brother own property, have investigated possible links between the serial killer and women who were reported missing on their turf but have not filed charges.

On Long Island, he was living a double life — a Jekyll-and-Hyde lifestyle that he hid for decades.

The two lives collided in April 2023, when Heuermann was arrested outside Manhattan office and charged with three of the cold case killings, with the rest added later thanks to DNA testsing.

He pleaded guilty this month to butchering Amber Lynn Costello, 27; Megan Waterman, 22; Melissa Barthelemy, 24; and Maureen Brainard-Barnes, 25, who were famously known as the “Gilgo Four” — as well as Valerie Mack, 24; Jessica Taylor, 20; and Sandra Costilla, 28, the first victim killed in 1993.

He also copped to killing Karen Vergata, 34, whose 1996 murder had not previously been linked to him.

Heuermann killed Costilla three years before he married Ellerup. He murdered his second victim, Vergata, in 1996 while his pregnant wife was in Sweden planning their wedding — then dismembered her inside the kill room.

The murders continued until daughter Victoria Heurmann was 13 years old, with the killer claiming to Winter he stopped killing because he wasn’t getting an adrenaline rush any more.

Heuermann is due to be sentenced to three life terms on June 17.