When Patricia Highsmith looked in the mirror, she saw both a lover and a killer. Early on, the reflected face had a fetching feline allure, but out of sight another facet of Highsmith seemed to belong, she said in 1942, in “a terrible other world of hell and the unknown”. As she aged, what she saw through the “evil distorting lens of my eye” changed: now a gravel-voiced, fire-breathing ogre stared back. Highsmith knew that there are always “two people in each person”, and in 1953 a nightmare confirmed this duality. She dreamed that she was incinerating a naked girl who shivered in a wooden bathtub; the funeral pyre was set with papers, presumably Highsmith’s manuscripts. Waking up, she admitted: “I had two identities: the victim and the murderer.” The characters in Highsmith’s novels accordingly come in pairs, doubles who are casualties of a fracture in what she called “the universal law of oneness”. Upright Guy and devious Bruno in Strangers on a Train begin as opposites but end as psychic twins after they exchange homicides. Tom in The Talented Mr Ripley kills the alluring Dickie, then assumes his identity. In the lesbian romance The Price of Salt, matronly Carol and girlish Therese merge, then are sundered by social disapproval: murders, which for Highsmith were “a kind of making love”, are here replaced by orgasms. Highsmith regarded writing, liquor and sex as her addictive vices, and like a true decadent she valued sickness as an aesthetic boon: she wrote The Price of Salt while ill with chickenpox, and believed that the fever woozily sweetened her prose. Her diaries, unearthed in linen closets after her death in 1995, record alcoholic binges and erotic misadventures, casually noting the suicide attempts of women she rejects or betrays; she often encrypts the entries in foreign languages, perhaps to distance and disown her conduct. In her notebooks, a more lucid Highsmith incisively analyses her own neuroses and ponders the physics and metaphysics of a world blown apart in the 1940s by nuclear fission. “God and the devil,” she suggests in one entry, “dance hand in hand around every atom.” Those positive and negative energies continue their dispute inside her self-divided individuals, who think of love as radioactivity released by an explosion: Carol in The Price of Salt believes that Therese has been “flung out of space” to land in her lap. The binary split that most tormented Highsmith was a matter of gender. As a primal practical joke, God or his diabolical double segregated us into male and female. Little Pat, however, protested by announcing at the age of 12 that she was a boy mistakenly assigned to a girl’s body. In 1948 Highsmith informs her diary that “I want to change my sex” and plaintively asks: “Is that possible?” At the time it wasn’t; instead, she placed her trust in the crude truisms of pop psychology and decided that she suffered from penis envy. Once at least she overcame her imagined deficiency: fantasising about her current girlfriend, she reports that “I had to go to the bathroom to relieve myself of a large erection”. In a meta-kinky episode she titillates the gay photographer Rolf Tietgens by posing as a male pin-up. “Yes, he fancies me a boy,” she smirks, “because my body is hard & straight.” She did make valiant efforts to smooch with men, though it felt like kissing a flounder; she was even fitted with a diaphragm, which she describes as “the sign of the whore”. Bedding other women, Highsmith saddled up to assume masculine duties. “Kissed her masterfully,” she says with a roué’s swagger; on another occasion she regrets that her partner “couldn’t tell when I came”, then disposes of maidenly pretence by guessing that “she must have done it before!” At her most promiscuous she resembles a male libertine keeping numerical score of his successes. Mozart’s Don Giovanni has his encyclopaedic list, and Highsmith drew up a chart that graded her lovers. Flagging a little, she asks: “My God, how many women do I want?”; like a man whose hydraulics are overtaxed, she protests”: “I’m not a machine!” Such relentless tom-catting suggests that Highsmith’s motive might have been a will to power as much as amorous craving. “Having an automobile,” she declares at the age of 20, “is like having your own woman.” This was truer still in reverse, since the female bodies she manhandled were vehicles driven by her. Gratification intensified if her conquests defiled themselves verbally: “when she uses dirty words, she excites me!” she says after a pleasingly X-rated romp. In 1968, enraged by Jacqueline Kennedy’s venal marriage to Onassis, Highsmith snarls that “women will sleep with anything”. Yet her problem was less misogyny than misanthropy: like Ripley, the psychopathic aesthete in her five novels about him, she was disgusted by the human species. On a walk through Central Park in 1942, she reduces her fellow New Yorkers to “amorphous subaqueous organisms”. “I am not interested in people, knowing them,” she sniffs in another diary entry. That ought to be a disqualification for a novelist, but Highsmith’s stark existential parables overlook our complex affiliations with friends, family and society at large. She found people intolerable because they were her versions of herself, and blocked her view of them by designing a virtually windowless house in Switzerland, where she spent her last years among the gaunt mountains. Acquaintances, discouraged from visiting, likened her home to Hitler’s bunker. Anxious to outgrow lowly humankind, in the diaries Highsmith wishes that she could be a giant, and in a more grandiose reverie fancies becoming God. “One needs a wife,” she offhandedly remarks when settling into a new home, but for domestic company she preferred gastropods: she kept snails as pets, and smuggled them into England from France hidden in her bra, as if she were suckling them. Repellently slimy, they were fortified inside their shells like the increasingly crustacean Highsmith herself. Imagining a nuclear apocalypse, she permits snails to survive and to repopulate the irradiated earth. Many of her short stories are beast fables that rejig relations between humans and species we think of as inferior. In one, a boy butchers his mother after she cooks a tortoise he has befriended; in another, the owner of a German shepherd kills himself because he feels shamed by the dog’s noble demeanour; a third tracks the career of a snooty cockroach, proud of the luxurious addresses it has infested. Highsmith admired animals because they were incapable of murder: on a visit to Ascona she watches a slim snake elegantly engorge a live frog – a natural process, not one of Ripley’s gratuitous executions. She came closest to empathising with another creature when she dedicated a novel to her cat, Spider, only to quash sentimental tenderness by acknowledging in a poem that her yellow-eyed pet can’t read the tribute. The dualism that bedevilled Highsmith drives a wedge through this enormous volume. Her diaries, scribbled by a dissipated tomboy who aged into a cantankerous dragon, are ribald, chaotic, and – as her prejudices calcify – often nasty. The notebooks are clearer-headed, containing blissful love lyrics, airy surveys of European landscapes, and bold philosophical meditations. Highsmith, who wondered at the aplomb with which her snails slithered along the edges of razorblades, was intrigued by Kierkegaard’s account of the tightrope-treading anxiety with which we advance through life. However, she refused to follow the Danish theologian when he made his “leap of faith” into the unknown; instead, she challenged God by asking: “Have you the courage to show me hell?” Her chosen deities were savage and deathly, and one entry, luxuriating in sin, ends with a pagan invocation – “O Siva! O Pluto! O Saturn! O Hecate!” But she cautiously appeased the established religion, and while living in a village on the Hudson River she sang in the local Presbyterian church choir. Ripley collects paintings and plays the harpsichord. Highsmith both painted and sculpted – typically she did her best carving when angry, using her implements as weapons – and was also enraptured by music. In the notebooks she overhears a preview of the afterlife in Mozart’s Requiem, giddily rotates to Viennese waltzes, and rebukes the “sacrilege” of a young man who wants to have synchronised sex while listening to Wagner’s Tristan und Isolde. “Music,” she concludes in 1973, “establishes the fact that life is not real” – for me, the most striking sentence in these thousand pages. Elsewhere, she undercuts that romantic sublimation. A 1949 diary entry asserts that “there is no reality, only a system of expedient behaviour... by which people have come to live”. The contradictory formulations are voiced by the two people who were resident inside Highsmith. One side of her wishfully transcends the world, like Carol and Therese in their brief flight from conformity. The other, imitating the vengeful Ripley, condemns the world as a mausoleum and adds to its stock of corpses. It’s a pity that Highsmith’s daring, disturbing novels have been upstaged by a few admittedly excellent film adaptations – Hitchcock’s Strangers on a Train, Anthony Minghella’s The Talented Mr Ripley, Wim Wenders’s The American Friend (based on Ripley’s Game), and Todd Haynes’s Carol. Although some of her work first appeared in the grubby pages of Ellery Queen’s Mystery Magazine, her models, she always insisted, were Dostoevsky and Kafka, and the notebooks demonstrate that she belongs in their vicinity. But you should brace yourself before reading: Highsmith likens herself to “a steel needle”, and her insights puncture complacency as if piercing flesh. She is the murderer, and we are all the victims.