PATRICIA HIGHSMITH was a character. An industrious drinker who was painfully shy, loved cats and once noted how easy it was to hate the human race; a bed-hopping romantic, a lesbian who despised women, a liberal who hated Jews, a woman who once described murder as “a kind of making love”. She was, in short, wholly unsuited to everyday life.

But she could write a mean story. Tom Ripley, the charming, murdering anti-hero of The Talented Mr Ripley, is her best known creation but it is in her short stories that the appetite for the cruel and macabre is best fed.

This collection of mostly unpublished stories, gleaned from the masses of diaries and notebooks recovered after her death in 1995, is a treat.

The discovery of her journals (or “cahiers”) reveals how much of herself she put into her work and where she found the inspiration for her most disturbing characters — the wandering souls who inhabit that no-man’s land beyond morality. In her life, she evaded personal questions, but her papers say it clear: “C’est moi!”

She describes writing as an outlet for her fantasies, as well as a means of avenging herself on former lovers (Marijane Meaker said that she was knifed to death — “for several pages” — at the end of The Cry of the Owl).

Of the Ripley novels, Highsmith said: “I often had the feeling that Ripley was writing”, and she added his name to certificates when she won prizes. A wry joke, perhaps, but the diaries suggest that it was only by writing of the mad and the homicidal that she stopped herself from joining them.

The individual’s (often wilful) dislocation from reality is a theme throughout these stories. In The Great Cardhouse, the protagonist Lucien stands stunned: “No one seemed to realise the catastrophe that was taking place inside him. He had a vision of himself without his artifices, without his arrogant faith in his infallibility — a piece of a man, unable even to stand upright, a miserable fragment. For a few moments, Lucien’s spirit bore the full weight of reality and almost broke beneath it.”

Again and again the characters’ anger at being faced with themselves is directed outwards. As Andrew, the confidence trickster of A Dangerous Hobby, throttles Miss Wooster, “bumping her head again and again on the floor”, he feels “a consciousness that the woman he was attacking had betrayed him, stripped him of decency, had filled him with an intolerable shame”.

Even at moments of crisis in minds that are mostly sordid, chaotic and emotionally disturbed, Highsmith remains calm, measured and, crucially, unobtrusive. There is no melodrama, just a seamless combination of realism, the abnormal and the delusional that take us to a place quite devoid of moral and emotional landmarks. We are marooned, with nothing to cling to.

Highsmith’s prose is lean, translucent, compact, direct and, crucially, neutral to the horror it depicts and indifferent to our attempts at rationalisation. These stories present the darkest of human endeavours, but never try to explain, or even to judge them.

Her characters are usually described as amoral, but this suggests that these people are outside humanity as well as morals. They are not. They are everyday people doing unspeakable things in such an everyday way that you start, bit by bit, to realise that it could be you.

Highsmith’s great trick is to show the world through the eyes of her most despicable characters. The details can be so banal — and the prose so neutral and direct — that the reader begins to identify with that person and to suspect that they are not so abnormal after all. This is why her stories are so haunting — because, like a Highsmith character, you realise that there is no way of escaping what humans are. Except, perhaps, by suicide.

Times Recommends

• The 100 Best Books of the Decade
• The Times picks the 50 best paperbacks of 2009
• The conversation: James Ellroy