The man who blew away John Lennon nearly 25 years ago says he considered himself a "white knight" on an unstoppable mission the night he pumped five bullets into the back of the beloved Beatles star.
It was a predestined act, Mark David Chapman reveals, that would make the world remember him as a somebody, and not the pathetic loser he had been all his life.
"It was like a train, a runaway train — there was no stopping it," Chapman says on this Friday's "Dateline NBC," which airs at 8 p.m. "Nothing could have stopped me ... This was a white-knight kind of a thing, a crusade."
Dec. 8 will mark 25 years since Chapman approached the legendary rocker in front of the Dakota, calmly raised a gun to his back and fired repeatedly.
And every day, fans gather at nearby Strawberry Fields, the memorial Yoko Ono built in Central Park, to honor the Liverpool-born legend, who would have turned 65 this year.
In exclusive audiotapes of Chapman, recorded in 1991 and 1992, the pudgy-faced madman says he was in his apartment thinking about the Fab Four when he hatched his diabolical plan.
"I was sitting cross-legged on the carpet ... And I remember opening up the 'Sgt. Pepper' album," Chapman says on the tape.
"And 'The Catcher in the Rye' was very prominent ... And I remember saying in my mind, 'What if I killed him?'
"And I remember thinking perhaps my identity would be found in the killing of John Lennon."
Before that moment, Chapman recalls, he had seen himself as a pathetic loser whom nobody in the world cared about and who would die without achieving any fame.
And his opposite, the person whom he hated the most because of his ability to communicate with millions, was Lennon.
"There was a successful man who kind of had the world on a chain, so to speak, and there I was, not even a link of that chain, just a person who had no personality," Chapman says.
"And something in me just broke."
On the night of Dec. 8, 1980, just weeks after the release of his "Double Fantasy" comeback album, Lennon, then 40, was returning home to the Dakota on West 72nd Street at Central Park West when Chapman, who had been lurking nearby, approached.
"It's a little bit cold and there's a little wind, and somehow I knew that this was it, that this was the day. And all of a sudden from way across Central Park West I see the limo. And I know that, that it's him," Chapman says.
"I have this incredible feeling. And John Lennon's car pulled up. I heard a voice in my head saying 'Do it! Do it! Do it!'
"And as he passed me, I pulled out the gun, aimed at his back and pulled the trigger five times in succession."
Cops quickly nabbed Chapman at the scene and he readily confessed to his evil deed — a deed he says was inevitable.
"I was under total compulsion. I'm thoroughly convinced in my, in my conscience and in my heart that there was nothing I could do beyond that point to help myself — totally convinced of that," Chapman says.
"You have to understand, I don't think I truly thought I was doing anything evil. I thought I was good."
Stephan Spiro, the officer who busted Chapman, tells "Dateline's" Hoda Kotb the arrest "was strange."
"I've arrested people for many years," he says. "And nobody ever gives themselves up, whether they're alone or with somebody or — you know?"
It later emerged Chapman had seen Lennon and Ono leaving the Dakota earlier in the day and had even gotten Lennon to autograph an album for him.
In October 2004, Chapman was denied parole for a third time at Attica state prison, where he's serving 20 years to life, telling officials he deserved exactly what he got "because of the pain and suffering I caused. I deserve nothing." The 50-year-old murderer is not eligible for parole again until next year.
There are those who knew Chapman before the shooting who still can't believe he was capable of assassinating the rock 'n' roll icon, like his high-school sweetheart Jessica Blakenship, who was engaged to marry the madman but broke up with him in college.
Speaking publicly about the case for the first time, Blakenship tells the show:
"He was a great guy ... It's kind of like, if you think back to when you were in high school, and you think about the person that is least likely to commit a violent act, that's who Mark was.
"And that's part of what I really loved about him, was he was so gentle. Mental illness is all I can say. That's my only explanation ... And I do believe in evil. I do believe in the devil. I'm sure he had his hand in it. You know? In influencing him."