Jacques-Louis David's The Death of Marat is a deceptively simple image of a real-life murder. But a closer look at David's iconic painting reveals the political messages contained within.

Great art makes us do a double take. It makes us look, then look again. Take The Death of Marat, 1793, perhaps the most famous crime scene depiction of the past 250 years. At first glance, the portrayal of the murdered body of the French revolutionary Jean-Paul Marat, stabbed to death in his bath on 13 July 1793, could hardly be simpler. The slain journalist, who had agitated for the execution of King Louis XVI, slumps towards us – his body framed by the vast flickering emptiness that stretches above him.

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Lean in closer, however, and Jacques-Louis David's iconic painting begins to break down into a complex puzzle of double details that unsettle the bottom half of the canvas – two quills, two dates, two letters, two absent women, two boxes, two signatures, two dead bodies. The cacophony of contrary clues draws us in, transforming us from passive observers of a straightforward snapshot of history to forensic detectives actively engaged in solving a deeper mystery, one in which the artist himself is suspected of having tampered with the evidence.

Everywhere you look in The Death of Marat, one of the masterpieces featured in a major exhibition of David's work at the Louvre in Paris, there is proof of the artist's dual determination to create both an intimate personal elegy for a murdered friend, whose radical politics the artist shared, as well as a piece of potent public propaganda. In David's hands, Marat is much more than simply a Jacobin journalist into whose chest a French woman, Charlotte Corday, plunged a kitchen knife, believing he was poisoning public discourse. Marat is glorified: a second Christ.

David's portrait exalts Marat, transfiguring him from a sickly real-life person, who required lengthy medicinal baths to soothe a chronic skin disease, into a sacrificed secular Messiah. To amplify that elevation from infirmed mortal to mystical martyr, David laces his painting with decodable ciphers and echoes of art history that keep our eyes firmly fixed on the myth he is weaving before them. So implicated is the artist in the choreography of the scene, it is easy to see how Sébastien Allard, curator of the Louvre exhibition, could reach the conclusion in his essay for the catalogue that "the monument David erects to Marat is also a monument that he builds for himself… Marat acts with his pen, the painter with his brushes".

Our gaze is torn in two directions as it tries to trace the curiously contrary activities of the dead man's moribund hands. In Marat's right hand we find the quill with which he was writing when stabbed with the pearl-handled knife that lies only inches away. Knuckles to the floor, that hand dangles lifelessly downward in a manner that recalls Christ's drooping arms in both Michelangelo's monumental marble sculpture, Pietà, and in Caravaggio's affecting painting The Entombment of Christ, 1603-4. Meanwhile, Marat's left hand, rigid with rigor mortis, steadies a blood-smudged letter from the assassin, suggesting an entirely different focus of his attention. One hand clings to life, the other succumbs to death. Between these two diverging gestures, the painting's spirit swivels, flexing forever between the world of the living and the world of the dead – this one and the next.

Compounding that friction between the restless flux and sombre stillness of........

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