Stone verdict: He shambled through from day to day, gripped by a desperate need for heroin

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From the age of nine, Michael Stone lived a dingy half-life, shambling through each day, long before he was found guilty of a crime of almost unimaginable violence.

He was known to have a psychopathic personality disorder. Police files contain details of his threats and torture fantasies, stretching back for years.

During both his trials the juries involved were not made aware of his medical record and his propensity for violence. But yesterday, after the guilty verdict, as his previous convictions were revealed, one female juror started to cry.

Stone's childhood was split between the home of his mother, Jane Standen, and the man he saw as his father, Peter Stone. He spent time between the ages of 10 and 16 at several children's homes in the area of the killings. By 16, Stone had left behind his care and school, and entered the outside world equipped with few qualifications and little ambition. He moved to Gillingham, Kent, where he moved quickly into the world of crime and drugs.

By his mid-20s, Stone had served short prison terms for theft and was beginning to be gripped by a heroin addiction.

In 1981, he was jailed for two years for attacking a homosexual man with a hammer, and robbing him. Two years later, he was given four and a half years for stabbing a man and attacking a policeman who was left almost blind. In 1987, he received 10 years for armed robbery on a building society in Brighton. While inside he attacked several prison officers but was out after five years.

During his time in jail, Stone received attention for a violent personality disorder, but subsequent drug treatment on his release was thrown awry because of his heroin use.

To pay for his habit, he mugged people at cash machines and broke in to garden sheds to steal. Stone was known to have stolen lawnmowers from isolated homes in Kent; one was taken in Chillenden on the day the Russells were murdered.

Otherwise his life revolved around squalid bedsits. His £150-a-day drug addiction meant that he injected five or six times a day. He had been using drugs for so long that most of his veins had collapsed.

One of Stone's former friends, known only as Mick, said: "All Stone's friends were the same. They would just sit in his flat shooting up or be out thieving and robbing. But Stone was a bit worse. He had a real temper on him. When he lost it, he really lost it."

Stone always denied destroying the Russell family. But for months prior to the murders, his mental state had been deteriorating and he had even begged his psychiatric nurse, Margaret Stewart, to get him admitted to hospital. Mrs Stewart told the police – in evidence never put before the jury – that, five days before the murders, Stone had said he felt he was going to kill people, making lists of people he wanted to die, including his probation officer whom he had threatened before. But the only hospital place available was not considered secure enough.

Stone was also a Kent police informant and had met his handler at a pub in the county the day after the murders as well as four days before them. His troubled past meant that, even without scientific evidence or a positive identification to link him to the murder scene, police instantly thought they were on the right track to finding the Russells' killer.