The assassination of John Lennon on a winter’s night in New York, is burned into the memory of almost everyone over a certain age in the western world. For millions of fans, that senseless moment seemed to signify the end of an era. To Andrew Piddington, the director of a new film about Lennon’s killer, Mark Chapman, it marked the beginning of an obsession.

After several years researching the minutiae of Chapman’s life, Piddington is finally ready to unveil The Killing of John Lennon, the first biopic to offer an insight into the twisted mind of the notorious killer, at the Edinburgh International Film Festival.

From the start Piddington, a veteran television director, knew the decision to make a film about the assassination on December 8, 1980 was not going to be universally popular.

“The killing of John Lennon is a volatile subject that still generates a lot of emotion,” he admits. “In taking on such a subject through the eyes of his killer we are almost inviting adverse criticism and scorn. But I didn’t want to take any soft options. I know the realism could upset people, but I feel we have a responsibility to the truth.”

It seems unlikely that Yoko Ono, Lennon’s widow, will appreciate Piddington’s efforts. In a recent interview she said documentaries and films about Chapman — such as Chapter 27, a second film about Chapman to be directed by Jarrett Schaeffer, and planned for 2007 — bring back painful memories.

For Piddington, the key difference between this film and its rivals, is its truthfulness. “I didn’t want to misrepresent the psychosis of the man in any way,” he says. “It in no way exonerates or sympathises with him.”

The script is a work of verbatim screenwriting extracted from police statements, interviews, journals and psychiatrists’ transcripts, and recreated in a chilling voice-over by Jonas Ball, the actor playing the killer.

Piddington combed New York’s junk shops, searching out Chapman’s dark glasses, Hawaiian shirt and a period copy of The Catcher in the Rye, which Chapman was clutching when he shot Lennon.

Location shooting was arranged with the same precision. The tiny low-budget film crew went to Decatur, Georgia, where Chapman attended Columbia High School, and to Hawaii, where he purchased the Charter Arms .38 revolver he used to kill Lennon.

But it is the final section of the film, after Chapman had been charged with second-degree murder and sent to Attica state prison, near Buffalo, in upstate New York for 20 years, which Piddington believes is the most chilling.

Cold and remorseless, Chapman’s pride in becoming “the biggest nobody who killed the biggest somebody” is revealed with hammer blows.

Perhaps most moving is the exploration of just how random an act of violence this ultimately was. In interviews with psychiatrists, Chapman admitted that Lennon, was merely one individual on a long list of celebrities who could have become his victims.

“In the end this film explores three tragedies,” says Piddington. “There is the tragedy of John Lennon and the fact that we will never know what he could have achieved.

“There is also the tragedy of this 25-year-old who needed psychological help and didn’t get it in time. And there is the tragedy of their families who have to come to terms with what happened. To me, therein lies the desolation.”

The Killing of John Lennon opens at the Edinburgh International Film Festival at Cineworld on Tuesday at 9.30pm and Wednesday at 5.30pm

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