On Monday, a British jury convicted the 42-year-old Zardad of torture and hostage-taking in his country, in what prosecutors called the first trial in Britain of someone accused of torture in another country.
"By securing this conviction, the Crown Prosecution Service has shown there is no hiding place here for torturers and hostage-takers," said Director of Public Prosecutions Ken Macdonald.
"Our lawyers have worked relentlessly to prepare this case -including visits to Afghanistan - and have overcome the difficulties of proving crimes committed in another country over 10 years ago."
Zardad will be sentenced on Tuesday. Zardad came to London on a fake passport in 1998 and was managing a pizza restaurant in the capital when he was arrested.
He denied the charges, including conspiracy to torture and conspiracy to take hostages in an area outside Kabul between 31 December 1991 and 30 September 1996.
But, giving evidence via video link from the British Embassy in Kabul, his victims told of torture and abuse.
One witness said a man known as "Zardad's dog" had bitten a man at a checkpoint because he wasn't handing out grapes fast enough.
Another witness said he was held for months and was beaten so often that his family didn't recognise him. A boy said he had seen his father tortured and his ear cut at the hands of Zardad's men.
Prosecutors said from 1991 to 1996, Zardad was in charge of the road from Kabul to Jalabadad in the Sarobi area, and his men set up checkpoints where they trapped opponents and abused them.
"By securing this conviction, the Crown Prosecution Service has shown there is no hiding place here for torturers and hostage-takers"
director of public prosecutions
"He and his soldiers wanted to create an atmosphere of fear and terror. He wanted a fearsome reputation for being cruel and merciless at his military checkpoints," prosecutor James Lewis told London's Old Bailey court.
The court heard that Zardad fled his homeland in 1998 in fear of his life, having fought both the invading Russians and the Taliban, first to to live as a refugee in Pakistan and then moved to Britain.
Soon after arriving in Britain, he gave an interview to British Broadcasting Corporation television, which drew attention to his whereabouts, and he was arrested by British anti-terrorist detectives.
"I have never killed anybody and if anybody in the whole of Afghanistan can produce that evidence against me then I will accept that crime," Zardad told police through an interpreter.
"I have never taken hostages, not even slapped anybody."
In November, an Old Bailey jury failed to reach a verdict at Zardad's first trial and the Crown Prosecution Service ordered a retrial. Legal officials estimate the prosecution cost British taxpayers 3 million pounds ($5.4 million).
Attorney General Lord Goldsmith told the first trial that the case was "the first of its kind, in any country, in international law, and certainly in English law, where offences of torture and hostage-taking have been prosecuted in circumstances such as this".
"It is unusual to try cases in the Old Bailey, indeed in any of our criminal courts, when the matters concerned did not occur in the United Kingdom. It is more unusual still to try matters when the defendant is not a British subject nor the victims British."
The British prosecution gathered
"However, there are some crimes which are so heinous ... that they can be tried in any country."
Peter Clarke, head of Scotland Yard's anti-terrorist branch, said the investigation into Zardad's activities was one of the most difficult his office had ever undertaken.
Zardad's victims had been "humiliated, frightened and intimidated" and were fearful of the consequences of speaking out.
"We had to find witnesses in remote parts of Afghanistan and give them the confidence to come forward to give evidence in a British court," Clarke said.
The New York-based Human Rights Watch welcomed the conviction, saying that while Zardad was just a medium-sized fish, it set an important precedent.
"It is a big deal. It is an important landmark decision because it represents the first case of an Afghan war criminal who has been found guilty in a legitimate court and it shows that such a thing is possible," said John Sifton, an Afghan specialist for the human rights agency.
"It is difficult and it is expensive, but it is possible," Sifton told reporters.
The group released a report earlier this month titled "Afghanistan: Bring War Criminals to Justice. Special Court Needed For Past Atrocities".