The Greeks called those who were ignorant of Greek “barbarians” because they seemed to babble meaningless sounds. We smile at this insularity, but don’t we do something similar by using quite pointed (often foreign) words for certain unacceptable kinds of behavior? I am thinking of words like thug, hooligan, desperado, bandit, assassin, ghoul, and the like, all appropriated from other languages. Certain actions, it seems, could only be committed by those unacquainted with our pristine mores. The French have “bougre” (whence our “bugger”), a contemptuous reference to Bulgarians, Ukrainians condemn unruly upstarts as “Calvinists,” and I’m sure that other linguistic groups reserve specific terms of opprobrium drawn from alien tongues for their worst troublemakers.

In the case of thugs, the term pales before the reality. Of course, after September 11, most of our traditional bad guys fade into quaintness. Our old villains look clumsy and rather ridiculous beside the cold-blooded and cowardly mass murderers of that horrific day. This is not true, however, of the original thugs. For generations, these sly and beguiling serial killers plied their ghastly trade throughout India. The exact tally of their victims is unknown but undoubtedly reaches tens of thousands, all innocent, unsuspecting, ordinary individuals, most of whose murders still remain unrecorded and unsolved.

The word “thug” derives from the Hindi or Urdu word t’hag, which means “deception,” hence the epithet “deceivers” for the practitioners of Thuggee. Deceit, in fact, was their signal modus operandi. Their practice was to fall in with a group of travelers on the lawless roads of 19th-century India and slowly to gain the confidence of their new companions. By posing as fellow merchants who sought safety in numbers, they insistently charmed their intended victims, often entertaining them evening after evening around a common campfire with music and verse. Sometimes they accompanied their prey for weeks at a time.

When the moment was right, at a given signal, the chief thug might idly muse about what time it might be, and when all craned their necks to check the stars, he would sling a weighted silk noose around his principal victim’s throat. While his sidekicks held the startled man’s thrashing feet, he would strangle him in seconds. Afterward, one of the thugs thrust a dagger into the victim’s eyes to make sure he was really dead; he was then tossed into a pit, his belly slit to make sure that the gases of decomposition didn’t give away the location. They did the same to his retainers and even family, including small children, and the thugs then travelled on in search of new quarry. After a good haul, they might party all night in some secluded grove. They murdered the poor as well as the affluent and sometimes killed fellow travellers for as little as a single rupee.

This awful cult has been much studied. In the 19th century, growing fascination with the secretive killers gave rise to sensation, prompted by the 1839 thriller “Confessions of a Thug” (which Queen Victoria herself read with breathless suspense, commanding that the unbound pages be forwarded to her by the printer as soon as possible). Meadows Taylor, the author of this lurid potboiler (which remains gruesomely readable today), served in the Raj with Colonel William Sleeman, later famed for his extirpation of Thuggee, and knew whereof he wrote. In fact, though Meadows Taylor embroiders his narrative, it would be hard to overstate the murderousness and savagery of the Thugs.

Now the Cambridge-trained popular historian Mike Dash has written what is easily the best and most judicious account of this bizarre episode. In “Thug: The True Story of India’s Murderous Cult” (Granta Books, 356 pages, L20), just published in England, he surpasses every previous account, both in the thoroughness of his research and in the clarity and cogency of his narrative. Even better, Mr. Dash writes superbly. I read his book practically at one sitting (and have been having stealthy, silken nightmares ever since).

Colonel William Sleeman, who made it his life’s work to track down and bring the Thugs to justice, was truly horrified by the crimes he uncovered, but he was also motivated by a healthy sense of ambition. Mr. Dash’s portrait of this complex and noble officer, and of his dogged rise to celebrity, is one of the triumphs of his book. Sleeman had his limitations; like his countrymen, he exaggerated the role of religion in the Thugs’ bloody exploits. The English in India were woefully ignorant of Hinduism, and what they did know of it, as in the (rare) practice of suttee, left them aghast.

From Mr. Dash’s careful and well documented account, it becomes clear that the Thugs were motivated less by devotion to Kali, the bloodthirsty goddess (and patroness of Calcutta), than by an ugly mix of greed and superstition. In one interview Sleeman asked a Muslim thug named Dorgha whether they ever spared travelers who were poor, and the killer replied, “Let them go – never, never.” And Nasir, another Muslim thug, added, “How could we let them go? Is not a good omen the order from Heaven to kill them, and would it not be disobedience to let them go?”

In fact, as Mr. Dash shows, the overriding motive was simpler. Thugs came to experience a violent thrill in the pursuit and murder of their victims. One thug compared himself to an English big game hunter, a motivation that Sleeman and his fellow officers, avid hunters themselves, found weirdly comprehensible and even “sporting.” The Thugs themselves, by the way, ascribed their downfall not to Sleeman’s implacable campaign but to their own violation of ancient rules not to kill women and members of higher castes – unsporting behavior of the worst sort.

In the absence of forensic evidence or of witnesses, Sleeman could stop Thuggee only by using informers, known as “approvers,” who confessed their own crimes and implicated others. Though hundreds ended up on the gallows, the approvers themselves were pardoned and confined for life in special compounds where they learned useful skills such as weaving. I’ve long been fascinated by the sinister history of Thuggee, but I’m not quite sure why, other than that it’s a story with all the ingredients: ruthless and shadowy murderers, an exotic locale, and stubborn and brilliant sleuthing.

Mr. Dash’s enthralling account strips away all the usual political, social, and psychological explanations. Thuggee cannot be explained as a reaction to the Raj, an early “anti-colonialist” movement, because it antedated British occupation of India; moreover, the Thugs themselves never targeted English victims. He also clears away the sensationalistic image of the Thugs that even those who knew them best, like Meadows Taylor, tended to perpetuate. What we are left with is something radically fascinating but far uglier, more chilling, and more inexplicable, something that we prefer to dismiss: a pleasure, indeed, a virtual intoxication, evil purely for its own sake.