Herman Wallace, who has died aged 71, left a Louisiana prison on October 1st, where he had been detained in solitary confinement for the last 41 years. On the following Friday he died of cancer in New Orleans.

Wallace had been freed by a federal judge, who ruled that his original indictment in the killing of a prison guard had been unconstitutional. He had been one of the “Angola 3”, convicts whose solitary confinement at the Louisiana State Penitentiary in Angola, an 18,000-acre prison farm on the site of a former slave plantation, became a rallying point for advocates fighting abusive prison conditions worldwide.

Wallace had been serving a sentence for armed robbery when a prison officer, Brent Miller, was stabbed to death during a riot at Angola in April 1972. Wallace was one of three men indicted in the killing. He and Albert Woodfox were convicted in January 1974 and placed in solitary confinement, joining another prisoner there, Robert King. Amnesty published a report on them in 2011, and they were the subject of a documentary in which Brent Miller’s widow said: “If they did not do this – and I believe that they didn’t – they have been living a nightmare.”

George Kendall, a lawyer for Wallace, said his client’s original conviction was a travesty based on shoddy evidence, and that the men had been kept in solitary confinement because they had been members of the Black Panthers. Officials worried “that they would organise the prison”, he said. Even from solitary, Wallace worked to improve prison conditions and to press his own appeals.

Wallace’s cancer was detected in June. On October 1st, Judge Brian Jackson ordered that he be released from prison, adding that he could be retried. He was moved to the home of a friend and supporter. On October 3rd he was indicted again. District attorney Samuel C D’Aquilla said in an interview that he believed the evidence originally used to convict Wallace remained sufficient to convict him again. “We just felt that he was a murderer,” D’Aquilla said, adding, “I know he was old, I know he had medical problems, but when he committed a murder, he didn’t have medical problems.” Brent Miller, the murdered prison guard, he said, “didn’t get another chance” .

As Wallace lay dying, his friends did not speak of the indictment as they held a bedside vigil. “One of the final things that Herman said to us,” his lawyers said in a statement, “was, ‘I am free. I am free’.’’

Herman Wallace was born in 1941 in New Orleans, the fourth of eight children. His mother, Edna Clark Williams, who died in 1996, worked in the Orleans Parish Prison. He is survived by his longtime partner, Maria Hinds, and five sisters.