The Hon Violet Gibson shot two people at point-blank range, herself and Benito Mussolini. Both survived. After the first (attempted-suicidal) shooting, Violet, alive because the bullet had bounced off a rib, lived quietly in a convent in Rome, doing jigsaws with her Irish maid, until the day she set off for the Capitol with a gun in her pocket. After the second shooting Mussolini, alive because he turned his head just as Violet fired, set out for a triumphal visit to Libya with a sticking plaster on his nose. Meanwhile Violet was half-lynched, then dragged, badly battered, into a room containing the colossal marble foot of the Emperor Constantine, there to be revived with brandy before being dispatched to prison. It was the end of her life in the world.

In 1926, at the time of their bathetic encounter, Mussolini was a splendid figure of a man who liked to display his muscled torso shirtless. Violet was tiny (5ft 1in, and emaciated), unmarried and not much loved, 50 years old but looking 60, and odd enough in her behaviour to have been twice admitted to sanatoria for the mentally ill. The contrasts between would-be assassin and intended victim are straightforward and obvious, but in this oblique and subtle book Frances Stonor Saunders finds correspondences as well, tracing through their two lives thematic threads: nationalism, madness, religious fervour, politics as theatre and – above all – the instability of moral judgment.

To try to kill the fascist leader when the King of England had just awarded him the Order of the Bath, and British journalists were reporting gladly that his "trim handsome black-shirted lads" were doing a fine job of keeping down the Bolshies, might be the act of a madwoman. But 20 years later, once the Duce had been defeated and lynched in his turn, and hung up dead by his heels ("like a prosciutto", says Stonor Saunders, with wonderfully shocking precision), her friends suggested that Violet should surely be released. Too late. History might have endorsed her political judgment, but two decades in the madhouse had done nothing for her sanity. She belaboured fellow-patients with a broom-handle. She believed her moods created the weather. She never came out.

Violet's father was Lord Chancellor of Ireland. Stonor Saunders is able to track her early life through the society pages – at a wedding in chiffon and pink carnations, in pearl-embroidered satin at a Dublin ball. We meet several of her grand Anglo-Irish family, notably her brother Willie, an enthusiastic member of the Gaelic League who, when he came out to Italy to retrieve his sister, aroused the suspicions of the police by wandering around the Colosseum in a saffron-coloured kilt. (Rumour had it he kept a tortoise in his sporran.) But Violet remains elusive. We don't initially hear her voice, we don't know important things about her – not even the name of the fiancé whom she met in rackety artistic ­circles in Chelsea, and who shortly thereafter died.

Stonor Saunders approaches Violet indirectly, by writing about others. Maud Gonne and Bram Stoker give glimpses of Irish cultural life. Alice James evokes the lot of a clever but compulsorily idle young woman. Violet's mother becomes a Christian Scientist; Violet herself is impressed by theosophy. Stonor Saunders gives us a droll tour of fin-de-siècle mumbo-jumbo with anecdotes about WB Yeats, Virginia Woolf and Cornelius Vanderbilt. When Violet becomes a Catholic convert (a "pervert" in the telling idiom of the day), Antonia White and Ronald Knox illuminate the experience. Her pacifism is juxtaposed with Ottoline Morrell's and Sylvia Pankhurst's. It's a distancing technique, but also a broadening one – we are given not just a woman but a world – and, thanks to Stonor Saunders's beady eye for a quotation, it means that her own brisk prose becomes the setting for some gems of late Victorian orotundity, and that her narrative is a glinting patchwork of anecdotes and quips. ("Christ, if he had been woman, might have been nothing but a great complainer": Florence Nightingale.) It means, too, that when we hear Violet's voice at last, it rings out like a gunshot.

In her 40s, in 1916, Violet attended a Jesuit retreat. She kept notes; Stonor Saunders quotes from them. One verb tolls through her jottings: "mortify . . . mortified . . . mortification" and – lest there be any mistake – "Mortification means putting to death". Soon she was wandering around Kensington with a kitchen knife in her hand, having left her Bible open at the story of Abraham sacrificing Isaac. By the time she left for Rome, her closest friend Enid was pretty certain she intended to kill someone there. Enid thought the intended victim was probably the pope.

Mussolini's life is a great deal better documented, but Stonor Saunders keeps her distance from him as well. Her account of his career is not the résumé of an already oft-told story, but a quizzical analysis of it. She compares his grimaces and flamboyant gestures to those in Charcot's photographs of the insane. She shows how he rewrote the story of the attempts on his life as a kind of Passion, with himself as Christ. She gives a brief but trenchant summary of the history of fascist ­misogyny. "Fancy, a woman!" exclaimed Mussolini, aghast, upon ­being shot. He was ready, he said, for "a beautiful death", but Violet, one of the "old ugly repulsive women who come from abroad in groups", was absolutely not the kind of person he wanted to be killed by.

The two lives intersect, and then both, separately, undergo what Stonor Saunders calls "lockdown". Mussolini's government becomes ever more repressive. Poor Violet is repressed. Among the many evocative photographs in the book, there are two that are almost too sad to contemplate. One is of Vaslav Nijinsky – paunchy and balding after two decades of lockdown – jumping, a broken-spirited faun, in a hospital corridor. The other is of Violet in the asylum grounds, shapeless in a mackintosh, her face averted, with little birds feeding from her hand. The pose is that of Giotto's St Francis. Stonor Saunders, whose field of reference is wide, spots the resemblance, and suggests that Violet would have done so, too. Her doctors described her state of mind as "exalted". Violet might have accepted the epithet, and read it as meaning saint-like, uplifted, God's chosen instrument, his angel of death.

This book approaches a big subject by means of a small personal story, with numerous public ramifications. It's structure is accordingly bipartite, a linear narrative interrupted by essay­istic wide views. Two chapters stand out. One is a jazzily syncopated summary of the Roaring Twenties, the happy-crazy world-gone-mad of parties and sexual experimentation and "Nancy Astor doing cartwheels in the drawing room at Cliveden", which modulates into a plangent account of the miserable-crazy aftermath of private lives shattered and public life brutalised.

The second virtuoso passage recounts Violet's return from Italy. With the gracious manners of the ruling classes, Violet wrote a thank-you letter to her jailer – "Please be so kind as to offer my thanks also to the Director'" – then allowed her sister and a posse of police officers and nurses to escort her on to a train. Her sister watches her carefully as they pass through France (where she is, though she doesn't know it, free to escape). Arriving after dark in London she is taken by cab to ­Harley Street, examined by two ­different doctors (it is now nearly midnight), certified insane and driven straight to a Northamptonshire asylum, where she is washed, drugged and locked down for the rest of her natural life. Stonor Saunders's account of this ghastly journey is admirably terse. We are given no spurious attempt to guess at Violet's feelings, only the affectless language of the diagnoses, and the repeated blows of grim fact after grim fact.

In the asylum, and in the last chapters of this book, Violet finds her voice. There are letters, lots of them, to her family, to Winston Churchill, to Princess Elizabeth. It is the final pathos that these letters were never posted. In them she proposes plans for the ­betterment of the world – plans almost as grandiose and ill-founded as those of the man she tried to kill. Violet's lonely life was poignantly inconsequential: Mussolini's was full of world-historical bluster. Violet failed to kill two people. Mussolini, according to Stonor Saunders, caused at least three million deaths. Plaiting their two stories together in her vividly intelligent book, she adroitly shifts the balance, making the batty spinster the representative of a legion of outsiders, and showing the Duce as one whose hold on reality was as tenuous as hers.

Lucy Hughes-Hallett's books include Heroes: Saviours, Traitors and Supermen (Fourth Estate).