IT'S a thrilling moment.

A literary agent phones and says she loves your style, that your writing is original, that she utterly believes you've got inside the heads of your fictional characters. That, however, is the problem. The characters are Myra Hindley and Ian Brady and they're not fictional. It's a step too far for her. "The material," she pronounces, "is just too grim."

Her response to my novel, Myra, Beyond Saddleworth, was shared by approximately 50 other agents and publishers. At first I thought this was just the normal run of things for a new novelist. The UK publishing industry, despite contributing over £5 billion annually to the economy and releasing more books per head of population than anywhere else in the world, considers itself to be in difficulties, meltdown even. Some obscure writer comes along with a book that doesn't fit neatly into the predictable genres? Of course they're going to reject it.

But gradually I came to realise the rejections were to do with Myra Hindley herself. For nearly 50 years she has been seen as the personification of evil, her notorious mugshot a devil mask so threatening to our society that the cover designer of Myra, Beyond Saddleworth was able to extract just the frame of her bouffant blonde hair and make her instantly recognisable.

Why is Myra Hindley a taboo subject for fiction when the facts continue to fascinate the public? Since the moors murderers' trial in 1966, there have been countless newspaper and magazine articles, non-fiction books, and even two television dramas, one about Hindley's relationship with Lord Longford, the other a dramatisation of the background to the murders. People have an insatiable appetite for information about the extreme behaviour of serial killers. Only last week, Appropriate Adult, a TV drama about the Fred and Rosemary West murder trial, won three BAFTA awards.

Between 1963 and 1965, Myra Hindley and her lover, Ian Brady killed five young people, sexually assaulting four of them and burying the bodies on the moors. Hindley used her femininity to lure the children into their car, asking them either to help her find a glove or to carry something for her. The first victim, 16-year-old Pauline Reade, was a neighbour, but others were targeted at random because they were alone. John Kilbride, aged 12, was enticed into their car with the offer of sherry, before being sexually assaulted and strangled by Brady. This was also the fate of 12-year-old Keith Bennett, whose body has never been found and still lies out on Saddleworth Moor.

Perhaps the most notorious murder was of 10-year-old Lesley Ann Downey on Boxing Day 1964. Separated from her friends because she had spent all her money at a local fair, Lesley Ann was taken to Brady and Hindley's home, where she was stripped, raped and murdered. She was buried, her clothes at her feet, on the moor. The final murder was that of 17-year-old Edward Evans in October 1965. After picking him up in a Manchester gay bar, Brady and Hindley brought him home, where Brady attacked him with an axe and throttled him with electrical cord. His body was trussed up in plastic sheeting and shoved into a bedroom.

Hideous though these crimes were, they don't compare in scale to subsequent serial killers like Fred and Rosemary West, who killed at least 12 young women, including their own daughter; or like the Yorkshire Ripper, Peter Sutcliffe, who inspired incalculable terror in West Yorkshire as he killed 13 women over a period of six years and attacked many more.

But the moors murders have a mythical quality: they remain both burned into our consciousness and yet so ugly that we do not want to face them. Friedrich Nietzsche said we have art so that we don't die of the truth, but literary people who seemed to think Myra Hindley as a subject was art would make them die of the truth. One agent told me that although crime fiction examines violent and depraved behaviour, it is acceptable to a mass market because there's always a detective on hand to represent normality and order. The irony is that my novel is much less sensational than most detective novels. Its premise is that Hindley did not die when the authorities said but was released to a new life and new identity. The murders themselves appear only in a few flashbacks.

WHAT are the elements that make these particular crimes so powerful in people's minds? The first is the moorland setting, like the background to a dark fairytale. Brooding above a gaggle of little Lancashire villages dotted at its edges, Saddleworth Moor is bleak and blank, a desolate stretch of country containing nothing but rough grass and sheep and the occasional bird. The A635 to Holmfirth runs through it but is so rarely used that Brady and Hindley did not have to go far to bury their victims – Hollin Brown Knoll, where three bodies were found, is only yards off the road. Rationally, the public knew that the children were dead and no longer able to care, but to leave young ones buried out in this sad and lonely place seemed so heartless, so cruel, that people were disturbed at a visceral level.

Then too, Ian Brady made tape recordings of the torture and murder of at least one of his victims, Lesley Ann Downey. Many serial killers take trophies of their victims, pieces of their body or clothing. Perhaps the most notorious was Ed Gein, later immortalised in the Hitchcock thriller, Psycho. When police searched his Wisconsin farmhouse they found a chair and suit made of human skin, a bowl fashioned from a skull.

But for the public this florid behaviour seems proof of the murderer's madness, identifying him as a psychopath driven by uncontrollable urges and therefore absolving him of responsibility. By contrast there is something detached about the act of making tape recordings, which suggests control and, even more scarily, rationality. As part of my research I began corresponding with Ian Brady, who is held in Ashworth secure mental hospital outside Liverpool. It was clear this detachment, the need to be an observer, is an essential part of his personality. He described getting the steam train from Manchester and coming up to Glasgow on impulse one New Year. Brady, who grew up in the Gorbals but moved to Pollok with his adopted family, the Sloans, had packed whisky, brandy, Gauloises and a gun in his black leather briefcase. He ended up standing outside the Sloans' Templeland Road home, but not going in to see them. "I wandered and paused frequently till dawn, warding off the hard white frost with toasts from the [bottles in the] briefcase," he writes. "The privileged invisibility of a benign ghost of New Year Past, Present and Future." His later invisibility, when he was murdering young people without detection, was far from benign.

The Lesley Ann Downey tape so horrified people when it was played in court that it sealed the fate of both Brady and Hindley forever. The death penalty had been abolished while they were held on remand before trial, otherwise Hindley, and not Ruth Ellis, would surely have been the last woman to be executed in Britain.

The final element which keeps the moors murders embedded in people's minds nearly half a century afterwards is that Myra Hindley was a woman. Even now, after years of the feminist movement and growing female assertion, women serial killers are rare. Some we can neatly label and therefore file away mentally as comprehensible. They do not unsettle our whole view of the world. Aileen Wuornos, executed in Florida in 2002 for the murders of six men, had led a life of torment so baroque (abandonment, rape, brutal beatings and a schizophrenic father who hanged himself in prison), it's a wonder she killed so few. In film-maker Nick Broomfield's 2003 documentary, Aileen: Life And Death Of A Serial Killer, she was visibly unhinged and paranoid, leading any observer to wonder how she could possibly be judged sane.

We can understand such cases, but nothing in Myra Hindley's background explains what she did. Yes, her father was violent, though he seems to have turned his aggression on his wife more than his daughters. Yes, she lived with her grandmother rather than in the family home, but that was not so uncommon an arrangement in those days and her grandmother loved and spoiled her. Yes, she fell madly in love with Ian Brady, but still had an affair with another man while she was with him. None of it fitted the template we as a society provide to explain such extremes of behaviour.

Myra Hindley, it seems to me, was the first woman who killed as male killers do. Historically, women who murdered did so in the domestic setting, either for love or for money. Mary Ann Cotton, for example, poisoned 20 people, including her own children – for the insurance pay-out. She was hanged in Durham jail in 1893. But Hindley entered into a world of sex and sadism for no other reason than the pleasure she and her lover took in inflicting pain on others. She said later that their lovemaking was always at its peak after a murder.

This is a somewhat uncomfortable thought for our post-feminism society to accept. Although feminists have fought for women to be accepted as equal economically and domestically, there is still an ingrained belief that they are biologically programmed to love and nurture because they become mothers, which has led to the "men are bastards" strand of feminism and the idea that it is not politically correct to look at women as perpetrators rather than victims of violence. Scientifically there is some backing for this view: an experiment by anthropologist Desmond Morris showed that women's pupils dilated with enthusiasm and warmth at the sight of a baby, whether they were mothers or not, whereas men's only dilated if they had children themselves.

BUT motherhood itself does not ensure that women are loving and nurturing. East Yorkshire mother Linda Clappison was recently convicted of punching and hitting her children, locking them in rooms so cold that they needed treatment for frostbite, while Rosemary West's definition of motherhood included throttling her son Stephen till the blood vessels in his eyes burst; sexually assaulting her stepdaughter Anne-Marie and strapping her to a metal frame in the cellar; and, of course, helping bury her daughter Heather under the patio when Fred West had strangled her for resisting his sexual advances. They placed the barbecue above the spot where their daughter lay.

Why, then, has Rosemary West not become a symbol of evil in the same way as Myra Hindley? Is it because she had been sexually abused by her own father and was of below average intelligence? Or is it simply that Hindley was first, the first female in modern times to become famous for murdering for kicks? Her own claims of being afraid of Brady, of almost being a victim herself, were disbelieved by the majority and indeed by myself, so it was with astonishment that I read one agent's comment: "Myra Hindley provoked such strong reactions from so many people and continued to do so right up until her death, and I'm not sure how the public would respond to a book that was largely sympathetic to her."

Fiction, both in reading and writing, is an act of empathy, of exploration, but the fictional Myra I have created is not a sympathetic character. She's a petty pilferer, she betrays her younger woman lover by an affair with a priest, and she has a cynical and self-absorbed view of the world. On the plus side she has charisma and a black sense of humour, but that's hardly enough to class her as sympathetic.

Perhaps if I had made her remorseful it would have sugared the pill for publishers. The current literary sensation, Fifty Shades Of Grey, is about an innocent young woman in love with a sexual sadist, but the darkness of the theme is diluted by the fact that he just happens to be the most handsome man on the planet and the richest. There is a great deal of dissolving and melting inside on the road to sexual fulfilment as well as helicopter rides, private planes and elegant apartments. The book has sold more than 10 million copies and been printed in 37 countries, despite its Mills & Boonish prose.

I am fortunate that a small independent publisher, Wild Wolf, has been brave enough to take my novel on, but am left with the question: do we simply prefer to explain things away rather than try to explain them?

Myra, Beyond Saddleworth is published by Wild Wolf Publishing in paperback and e-book (£9.99/£2.99) on June 11