As Julian Assange evades arrest by taking refuge in the Ecuadorian embassy in Knightsbridge to escape extradition to Sweden, and possibly the US, British commentators have targeted him with shrill abuse. They almost froth with rage as they cite petty examples of his supposed gaucheness, egotism and appearance, as if these were criminal faults.

These criticisms tell one more about the conventionality and herd instinct of British opinion-makers than they do about Assange. Ignored, in all this, is his achievement as founder of WikiLeaks in publishing US government cables giving people across the world insight into how their governments really behave. Such public knowledge is the core of democracy because voters must be accurately informed if they are to be able to chose representatives to carry out their wishes.

Thanks to WikiLeaks, more information has become available about what the US and allied states are doing and thinking than ever before. The only competing revelations that come to mind were the publication by the victorious Bolsheviks in 1917 of secret treaties, including plans to carve up the Middle East by Britain and France. A more obvious parallel was the publication of the Pentagon Papers thanks to Daniel Ellsberg in 1971, revealing systematic lying by the Johnson administration about Vietnam. In similar fashion to Assange, Ellsberg was reviled by the US government and threatened with the severest punishment.

An extraordinary aspect of the campaign against Assange is that op-ed writers feel free to pump out thousands of words about his alleged faults, with never a mention of far more serious state crimes revealed by WikiLeaks. All these critics, and readers who agree them, should first switch on YouTube and watch a 17-minute video film taken by the crew of an Apache helicopter over east Baghdad on 12 July 2007. It shows the helicopter crew machine-gunning to death people on the ground in the belief that they are all armed insurgents. In fact, I cannot see any arms and what in one case was identified as a gun turned out to be the camera of a young Reuters' photographer, Namir Noor-Eldeen, who was killed along with his driver, Saeed Chmagh. The video shows the helicopter coming in for a second attack on a van that had stopped to pick up the dead and wounded. The driver was killed and two children wounded. "Ha! Ha! I hit 'em," shouts one of the US crewmen triumphantly. "Look at those dead bastards."

I was in Baghdad when the shooting took place and I remember at the time disbelieving, along with other journalists, the Pentagon's claim that the dead were all armed insurgents, but we could not prove it. Rebel gunmen did not amble about the streets in plain view when a US helicopter was nearby. The existence of a video of the killings became known, but the US Defense Department adamantly refused to release it under the Freedom of Information Act. The official story of what had happened would not have been effectively challenged if a US soldier, Bradley Manning, had not turned over the video to WikiLeaks, which released it in 2010.

The cables obtained by Wiki–Leaks were published later that year in five newspapers – The New York Times, The Guardian, Le Monde, Der Spiegel and El País – but the response to Assange himself was surprisingly mean-spirited and dismissive. Journalists seemed angry that their professional territory was being invaded by an Australian computer nerd who was doing their job. The British commentariat is notoriously club-like, conservative and hostile to those with different cultural and political norms.

But this in itself would not have been enough for so much of the media to declare open season on Assange. What created the difference were the allegations of rape made in Sweden. Allegations of rape destroy a reputation, however flimsy or non-existent the evidence or convincing the rebuttal. Assange has never really recovered from this. As for the suggestion that he exaggerates the chances of being extradited to the US from Sweden, this is surely very flip. Who would willingly take even a 5 per cent chance that their flight to Stockholm might result in 40 years' detention in a US prison cell?

Some adopt the official line that "lives had been put in danger" by the leaks. This lobby began to fall silent in 2011 when Pentagon officials admitted, off the record, that they had no evidence that anybody had come to harm.

A more dismissive response was that the WikiLeaks revelations were not as secret as all that and the papers accessed by Private Manning did not carry the most secret classification. Another point was made to me by a US diplomat in Kabul, where I was at the time of publication. He said: "We are not going to learn the biggest secrets from WikiLeaks because these have already been leaked by the White House, Pentagon or State Department."

In practice, the WikiLeaks documents are vastly and uniquely informative about what the US does and what it really thinks of the world in which we live. For instance, there is a cable sent by the US embassy in Kabul in 2009 describing Prime Minister Hamid Karzai as "a paranoid and weak individual unfamiliar with the basics of nation-building".

Specialists on Afghanistan commented that Karzai's failings were scarcely news. They missed the point that there is a vast difference between what is suspected by the outside world and what is confirmed by those with daily access to the Afghan leader. Here were senior and experienced US officials giving their true opinion of the man whom the Americans and British were fighting and dying to keep in power.

All governments indulge in a degree of hypocrisy between what they say in public and in private. When democratic openness about general actions and policies is demanded, they pretend they are facing a call for total transparency which would prevent effective government. This deliberate and self-serving inflation of popular demands is usually aimed at the concealment of failure and monopolising power.

What the US government wanted to keep quiet about in Afghanistan was not just an embarrassingly negative assessment of Karzai as their main local ally. It was that it had no credible local Afghan partner and therefore could not win the war against the Taliban.

Assange and WikeLeaks unmasked not diplomatic reticence in the interests of the smooth functioning of government, but duplicity to justify lost wars in which tens of thousand died. Recent history shows that this official secrecy, frequently aided by "embedding" journalists with armies, works all too well.

In Iraq, in the months before the US presidential election in 2004, foreign embassies in Baghdad all knew and reported that US soldiers were only clinging to islands of territory in a hostile land. But the Bush administration was able to persuade US voters that, on the contrary, it was fighting and winning a battle to establish democracy against the remnants of Saddam Hussein's regime and the adherents of Osama bin Laden.

State control of information and the ability to manipulate it makes the right to vote largely meaningless. That is why people like Julian Assange are so essential to democratic choice.