I am Shiva. " As the last line in a novel titled Kalki that might seem both baffling and mythologically muddled, and it is part of the genius of Gore Vidal, the author, who died earlier this week, that the novel is both, yet it possesses an undeniable and memorable power. A satire that enlisted an ancient Hindu myth to help skewer Seventies cults and culture, it somehow breaks through in the end to a colder, resonant place created by Vidal's peculiar combination of intellectual curiosity and abiding despair.

No one would argue that Kalki (1978) is among his best novels. It lacks the intense engagement with American history of the sequence of seven novels from Washington DC (1967) to The Golden Age (2000). It doesn't have the sexual comedy of Myra Breckinridge (1968) nor the courage of The City and the Pillar (1948), which is all the more remarkable as a coming-out novel both for its early year and also because Vidal had no interest in the gay writer label, yet still risked releasing the book. And it has none of the historical revisionism of Julian, based on the life of the last pagan Roman emperor, in whose defiant, yet doomed stance against Christianity, Vidal surely saw something of himself.

All these novels, in fact, contained something of Vidal. He was the scion of an elite American family, who was obsessed with American history and politics, twice running for political office, while also being quite open about his very active sexual life and the male partner he lived with for 53 years. Kalki has none of this. It is about James J Kelly, a former American soldier turned charismatic guru who declares he is Kalki, the last avatar of Vishnu, come to renew the world. He has set up a cult based in Kathmandu, where he lives with his wife, Lakshmi, who is presented as the ideal woman, and where he gathers a group of initiates who will help him in his task.

The subcontinental sections of the book are clichêd, showing Vidal at his weakest, but the novel picks up as Kelly's scheme becomes clear. There's a back-story of Soviet and American nuclear scientists and generals about to destroy the world with a neutron bomb, which gives Kelly his justification for saving the world - by first destroying it, just as the mythological Kalki is supposed to do at the end of Kali Yuga. Kelly's associates fly around the world distributing paper lotuses that they claim are part of a Hindu religious ritual, but in fact are impregnated with a virus created by Kelly that kills off the whole human race, barring Kelly, his wife and associates, who were all inoculated against it.

Kelly's plan is to repopulate the world with the children he will have with Lakshmi, helped by his associates, who were carefully chosen to be sterile themselves. But then Vidal springs a twist - one of the associates, a doctor, reveals that he is not sterile and that for a medical reason Lakshmi cannot bear Kelly's children, so it is only he who can father the new human race. Kelly kills the doctor which ends the chance of renewing mankind and the novel ends, 43 years later, with all the other characters having died of old age, and Kelly left as the last person alive. Which is when, conflating mythologies, he realises he was not Vishnu's last avatar renewing the world, but Shiva's, bringing its final destruction.

It is a story which has gained prescient power in time, as fears of bio-engineered destruction have grown. Just this last year has seen furious debate over whether to release academic studies on the H5N1 virus that could, theoretically, be used to create even more deadly versions, and also questions over whether intervening in the civil war in Syria could lead the Assad regime to deploy biological weapons. But while Vidal would have relished being right about the risks, what makes the end of Kalki so unsettling is less the technological threat, as the sense that he felt that the apocalyptic ending was perhaps justified - that Kelly as Kalki/Shiva was bringing a necessary end to a human race that no longer deserved what it had made of the world.

This nihilism was to lead Vidal into strange waters towards the end of his life. He formed a friendship over letters with Timothy McVeigh, the Oklahoma bomber. McVeigh invited Vidal to be one of the witnesses to his execution (Vidal wasn't able to make it). And while he didn't embrace the more insane 9/11 theories, he did suggest that the Bush government could have prevented it, but chose not to. Vidal did not support the terrorist attacks, but had a profound sense of the USA deserving it, in retribution for its imperial policies across the world.

It is some measure of Vidal's stature that such views, which in today's intensely partisan American culture would condemn most people holding them to total ostracism, never had that effect on him. On the contrary, as Andrew Sullivan wrote, many felt that "his willingness to court public outrage and disdain in defense of his ideas is a model for a public intellectual. " The wit, style and passion Vidal put into his arguments made him hard to resist, even when one disagreed, and there was always the uncomfortable sense that you never quite knew if he was serious or not, if he was challenging your intelligence, or mocking it.

It certainly made for great TV, and Vidal was one of the first intellectuals to realise its possibilities, quipping that, "I never miss a chance to have sex or appear on television. " But it wasn't just smart lines and devastating put-downs;beneath the style there was always a sense that he cared passionately, particularly about the America whose ideals he loved so deeply that he hated how politicians debased them. "The United States was founded by the brightest people in the country - and we haven't seen them since, " he once said. Over the years many causes he was early to champion, like anti-racism or gay rights, would win general acceptance, but it made him angrier for the other ways in which the US lagged, or went into reverse, as with the conservatism and military opportunism of the Reagan years.

This passion is what made Vidal the most brilliant of essayists, since he combined encyclopaedic knowledge on all kinds of writers and histories with this ability to make you realise why they mattered. Even a subject as abstruse as comparative religion could be made readable as he did in Creation, a novel which took the reader on a 5th century BCE tour from Persia to India and China, meeting along the way Zoroaster, Buddha, Mahavira, Lao Tsu, and Confucius, along with ancient Indian kings like Bimbisara and Ajatashatru. It was hardly surprising then that he would take a mythological concept like Kalki, which is obscure even in India, and fashion his novel around it.

Kalki the novel doesn't end entirely in despair. Kelly may have destroyed humans, but he recognises that the world still exists and regenerates itself. Nature spreads through the empty cities and species of apes might evolve into a sentient race that might make something new and better of the earth they inherit. Perhaps in a less apocalyptic way, Vidal did see some hope through the lies and decay of the world around him, trusting in a human instinct for truth, no matter how often betrayed. At the end of Julian, the last pagan philosopher, Libanius is left contemplating the emperor's lost promise: "With Julian, the light went out, and now nothing remains but to let the darkness come, and hope for a new sun and another day, born of time's mystery and man's love of light. "