You did not have to listen for too long to Julian Assange's half-educated condemnations of the American "military-industrial complex" to know that he was aching to betray better and braver people than he could ever be.

As soon as WikiLeaks received the State Department cables, Assange announced that the opponents of dictatorial regimes and movements were fair game. That the targets of the Taliban, for instance, were fighting a clerical-fascist force, which threatened every good liberal value, did not concern him. They had spoken to US diplomats. They had collaborated with the great Satan. Their safety was not his concern.

David Leigh and Luke Harding's history of WikiLeaks describes how journalists took Assange to Moro's, a classy Spanish restaurant in central London. A reporter worried that Assange would risk killing Afghans who had co-operated with American forces if he put US secrets online without taking the basic precaution of removing their names. "Well, they're informants," Assange replied. "So, if they get killed, they've got it coming to them. They deserve it." A silence fell on the table as the reporters realised that the man the gullible hailed as the pioneer of a new age of transparency was willing to hand death lists to psychopaths. They persuaded Assange to remove names before publishing the State Department Afghanistan cables. But Assange's disillusioned associates suggest that the failure to expose "informants" niggled in his mind.

It is hard to believe now, but honest people once worked for WikiLeaks for all the right reasons. Like me, they saw the site as a haven; a protected space where writers could publish stories that authoritarian censors and libel lawyers would otherwise have suppressed.

James Ball joined and thought that in his own small way he was making the world a better place. He realised that WikiLeaks was not what it seemed when an associate of Assange – a stocky man with a greying moustache, who called himself "Adam" – asked if he could pull out everything the State Department documents "had on the Jews". Ball discovered that "Adam" was Israel Shamir, a dangerous crank who uses six different names as he agitates among the antisemitic groups of the far right and far left. As well as signing up to the conspiracy theories of fascism, Shamir was happy to collaborate with Belarus's decayed Brezhnevian dictatorship. Leftwing tyranny, rightwing tyranny, as long as it was anti-western and anti-Israel, Shamir did not care.

Nor did Assange. He made Shamir WikiLeaks's representative in Russia and eastern Europe. Shamir praised the Belarusian dictatorship. He compared the pro-democracy protesters beaten and imprisoned by the KGB to football hooligans. On 19 December 2010, the Belarus-Telegraf, a state newspaper, said that WikiLeaks had allowed the dictatorship to identify the "organisers, instigators and rioters, including foreign ones" who had protested against rigged elections.

The proof of Assange and Shamir's treachery was strong but not conclusive. Given Shamir's history, there were reasonable grounds for fearing the worst. But even now, you cannot show beyond reasonable doubt that the state has charged this pro-democracy politician or that liberal artist with treason or collaborating with a foreign power because WikiLeaks named names.

One can say with certainty, however, that Assange's involvement with Shamir is enough to discredit his claim that he published the documents in full because my colleagues on the Guardian inadvertently revealed a link to a site he was meant to have taken down. WikiLeaks put the cables on the web last month with evident relish, and ever since I have been wondering who would be its first incontrovertible victim. China appeared a promising place to look. The authorities and pro-regime newspapers are going through the names of hundreds of dissidents and activists from ethnic minorities. To date, there have been no arrests, although in China, as elsewhere, the chilling effect WikiLeaks has spread has caused critics of the communists to bite their tongues.

In Ethiopia, however, Assange has already claimed his first scalp. Argaw Ashine fled the country last week after WikiLeaks revealed that the reporter had spoken to an official from the American embassy in Addis Ababa about the regime's plans to intimidate the independent press. WikiLeaks also revealed that a government official told Arshine about the planned assault on opposition journalists. Thus Assange and his colleagues not only endangered the journalist. They tipped off the cops that he had a source in the state apparatus.

Once we have repeated Orwell's line that "so much of leftwing thought is a kind of playing with fire by people who don't even know that fire is hot", there is work to do. First, there needs to be relentless pressure on the socialist socialites and haggard soixante-huitards who cheered Assange on. Bianca Jagger, Jemima Khan, John Pilger, Ken Loach and their like are fond of the egotistical slogan "not in my name." They are well-heeled and well-padded men and women who know no fear in their lives. Yet they are happy to let their names be used by Assange as he brings fear into the lives of others.

We need also to question the motives of the wider transparency movement. Anti-Americanism is one of its driving inspirations and helps explain its perfidies. If you believe that the American "military-industrial complex", Europe or Israel is the sole or main source of oppression, it is too easy to dismiss the victims of regimes whose excesses cannot be blamed on the west. Assange's former colleagues tell me that the infantile leftism of the 2000s is not the end of it. Never forget, they say, that Assange came from a backwater Queensland city named Townsville. He's a small-town boy desperate to make the world notice.

The grass or squealer usually blabs because he wants to settle scores or ingratiate himself with the authorities. Assange represents a new breed, which technology has enabled: the nark as show-off. The web made Assange famous. It allows him to monitor his celebrity – I am told that even the smallest blogpost about him rarely escapes his attention. When he sees that the audience is tiring, the web provides him with the means to publish new secrets and generate new headlines. Under the cover of holding power to account, Assange can revel in the power the web gives to put lives in danger and ensure he can be what he always wanted: the centre of attention.