You know that it’s coming all along, and, indeed, the title of the movie is a bit of a giveaway. But nonetheless, when the beloved Beatle, people’s poet and hippie icon John Lennon is shot two thirds of the way through The Killing of John Lennon it remains a profoundly shocking experience. Up until this point the writer-director Andrew Piddington’s woozy biographical trip deep inside the hyperactive mind of Lennon-killer Mark David Chapman has been something of a giddy impressionist whirl. We’ve gone from hometown Hawaii to New York and back again while the mercurial fantasist Chapman (played with terrifying verve by the newcomer Jonas Ball) has formulated his dastardly assassination plan with the sweet simplicity of a crackpot loon. The shooting, however, changes everything, and watching John Lennon getting blown to bloody chunks is a hard sell, even for nonBeatlemaniacs.

“I wanted people to be repulsed by it,” explains Piddington today, sipping a glass of wine in a quiet central London restaurant. “The act itself was so horrific. I mean, the man was virtually drained of blood. The first bullet exploded his left lung, just took it out completely. These were dumdum bullets – they hit and explode on contact. He never stood a chance. So, after a lot of thought, I decided to make the killing scene as realistic as possible. But, in fact, I should have had far more blood.” If Piddington seems unduly pragmatic, if not blasé, about the slaughter of a figure who is, to some, a 20th-century demigod, it is only because his argument is buttressed by years of fact-finding and meticulous research (the film is a four-year labour of love).

The 54-year-old director, a former TV veteran who earned his stripes on staple series such as Poirot, and on lavish two-parters such as The Dinosaur Hunters, says that he is aware that there is much in his movie that some might find upsetting, even controversial. The murder scene, for instance, might be read in some quarters as a second assassination, a double indignity heaped upon the sacred soul of Lennon. It’s no surprise that the entertainment bible Variety has said that the film is likely to set “tongues wagging”, while more forthright critics, such as the bloggers on the IMDB website, have accused the film of tasteless exploitation, saying that it was bathed in the blood of Lennon himself.

Yet surely the most controversial aspect of The Killing of John Lennon is that, despite its clearly exhaustive research into the fractured mind of Chapman (the dialogues and monologues in the film are made up entirely of Chapman’s own testimonies, transcripts and interviews), the film is subsequently sympathetic towards him, and gradually creates a portrait of a prototype stalker who was a hapless and unloved victim of his society, his family and his culture, rather than a homicidal nut-job. “I would totally disagree with that,” says Piddington. “My sympathy is certainly not with him. I don’t think that the film exonerates or condones what he did in any way. In fact, it probably does him a disservice.”

The film began, just like Chapman’s decision to assassinate Lennon, in a library. Here Piddington, an East London native, and the son of a concert pianist mother and a boxer father, casually chanced upon a book of conspiracy theories called Who Killed John Lennon? (Chapman, incidentally, back in Hawaii in 1979, found a John Lennon tome and decided to shoot the man for being a multimillionaire “phoney” who hypocritically preached about a world of “no possessions” in the song Imagine). The book’s incisive psychological profile of Chapman intrigued Piddington, as did one very filmic cue. “When I found out that Chapman was, just like [Ronald Reagan’s would-be assassin] John Hinkley, obsessed with Travis Bickle, and I saw very similar things within his story that related to Taxi Driver, that’s when I knew I had a big-screen movie.”

The film was budgeted at a modest £750,000 (“Less than a single episode of Poirot!” sighs Piddington), but took nearly four long years of on-again-off-again filming and funding to complete. Thanks to the guerrilla shooting style, location oddities included filming the preassassination scenes outside the famous Dakota Building in New York (once home to John and Yoko, now solely to the latter) and watching Ono’s black limousine arrive and depart with her inside, utterly oblivious to the macabre performance unfolding, literally, on her doorstep. Naturally, at this level of filming and financing, the chances of snagging a Beatles tune for the soundtrack were nonexistent. “Thosesongs cost up to £300,000 each to use in your movie,” says Piddington. [[ The finished film, with a subtle otherworldly score by Martin Kiszko, is an intense and propulsive portrait of a man on the edge that’s so far been critically adored, and has scooped the Special Jury Prize at New York’s own Tribeca Film Festival. And yet, for all the talk of the horrific murder itself, and for all the scenes afterwards of Chapman vainly soaking up his new-found celebrity status from prison, you can’t shake the feeling that the movie, by it’s very biographical nature and by the fact that it features Chapman in practically every frame, somehow wants us to sympathise with him.

“I would not want people to think that I’m sympathetic to Chapman,” reiterates Piddington. Only this time he takes a measured pause. “But at the same time,” he says, finally, “You have to have light and shade, otherwise there’s nowhere else to go.”

And besides, says Piddington, this obsession with controversy, and the offence caused to the Lennonites out there, is dulling the movie’s greatest resonance – namely that Chapman was nothing less than the progenitor of today’s celebrity culture. “John Lennon was the first rock’n’roll assassination,” he explains. “The names of Chapman and Lennon will always be linked together. Even Chapman’s quote, when he says, ‘I was a nobody until I killed the biggest somebody in the world,’ well, he has lifted that directly from the poster for Taxi Driver – ‘On every corner in every city there’s a nobody dreaming of being a somebody.’ Chapman has taken that notion and simply transferred it to himself, literally. So the idea of celebrity and being famous for nothing, which is so redolent in our society today, put its head above the ramparts for the very first time with this killing.”

In other words, the homicidal Mark Chapman is the father of every craven nobody who ever stood in front of a reality TV camera. It kind of makes sense.

In the meantime Chapman remains in solitary confinement (for his own protection), and has been there for 27 years now. Piddington didn’t contact Chapman for his side of the story, and says instead that he’d seen him interviewed by American TV hosts like Larry King and Barbara Walters and decided that talking to him would be worthless. “He’s only got one story to tell, as all narcissists have, and he tells the story slightly differently each time, depending on who he’s talking to.”

Nonetheless the film does feature a stark postscript that points out the inhumanity of Chapman’s incarceration – in solitary confinement for 27 years. The implication is that, perhaps, it’s time to consider Chapman for parole. “I think if he were ever to get parole then someone would put a bullet in his head,” says Piddington, coolly. Yes, but what would that mean for you, and for this whole Lennon-obsessed journey? “Well,” says Piddington, thinking hard, and digging deep. “It would be the end of the story.”

The Killing of John Lennon is on selected release from December 7

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