He mowed down an anti-apartheid leader, Chris Hani, with four bullets at close range: the courts decided on Monday to grant Janusz Walus parole after almost 30 years in prison.
"The Ministry of Justice and Correctional Services is ordered to place the applicant on parole," said Judge Raymond Zondo, president of the Constitutional Court, the highest court in the country, during a hearing broadcast on television.
The release must take place within 10 days.
Chris Hani, a 50-year-old communist leader and senior member of the armed wing of the African National Congress (ANC), was shot dead in the driveway of his garage on April 10, 1993, by Janusz Walus, a Polish immigrant linked to the white Afrikaner far right.
At the time, delicate negotiations with the white government for the country's first democratic elections were underway. The assassination exacerbated racial tensions and provoked violent riots in the townships of South Africa shaken by the last tremors of the racist regime. In a vibrant televised speech, Nelson Mandela called for calm.
Janusz Walus, now 69, had been sentenced to death but the new regime abolished the death penalty in 1994 and his sentence was commuted to life imprisonment.
In 2016, a court had already granted him early release. But that decision, criticized by the ruling ANC, was overturned the following year.
Janusz Walus had been eligible for parole for some 20 years. His previous applications had been systematically rejected.
Justice Ministry spokesman Chrispin Phiri told reporters outside the court in Johannesburg that this latest court decision will be "enforced.
"This judgment is diabolical, completely diabolical," reacted Limpho Hani, Chris Hani's widow, very angry. "An injustice has been committed today," lamented the leader of the Communist Party, Solly Mapaila.
The death of Chris Hani, in the pantheon of heroes of the anti-apartheid struggle, is commemorated every year in South Africa. His family has always vehemently opposed the release of his killer and accomplice, Clive Derby-Lewis.
This former Conservative MP, who provided the gun, died in November 2016. Suffering from lung cancer, he had been released the previous year for health reasons, after several unsuccessful applications for release.
In 1993, on that fatal Easter Saturday in a Johannesburg suburb, the murder of Martin Thembisile "Chris" Hani, a black idol, by two enraged white men, had traumatized the country.
After returning from exile three years earlier, Mr. Hani had already become one of the ANC's most popular leaders, one of the few to have developed an aura and influence on the ground, particularly among young township radicals, commensurate with his key role in the underground.