At the end of the 1996 cup final, troops appeared on the pitch as fans celebrated, and fired into the crowd.
Witnesses said that Saadi had been present, standing behind the red-capped soldiers.
Al-Ahli had won 1-0, and the common view was that he thought the crowd were jeering as a sign of opposition. "Nobody was booing, they were just celebrating," said Musbah Shengab, Al-Ahli's goalkeeper. "I lay on the pitch as the bullets went overhead."
Twenty people died.
Saadi escaped to Niger after the fall of his father's regime in 2011 but was extradited in March.
Also on trial is Saif al-Islam, who was caught escaping across the Sahara in November 2011, a month after his father's death, and Abdullah Senussi, Colonel Gaddafi's brother-in-law and security chief, accused of overseeing the prison massacre.
Senussi's son Mohammed was implicated with Saadi in one of the charge sheet's most inexplicable crimes: the disappearance in 2006 of Bashir al-Riani, once Libya's most celebrated striker and later a commentator.
After Tripoli fell, The Daily Telegraph found Riani's wife, Hamid Bin Mansour, who described her husband's final days. He had been taken up as a mascot by Saadi, until he tried to extricate himself from the relationship, alarmed by the heavy partying and threats of violence.
One day, a car arrived to "take him to dinner". A few days later, his son and brother-in-law found his body at the Tripoli Medical Centre. A policeman said it had been picked up from Saadi's beachside villa. In the years since, they learnt that the fatal blow was struck by Mohammed Senussi.
Saadi also flew women into Tripoli from around Europe, although another Al-Ahli footballer, Reda Thawargi, alleged he was jailed for refusing his homosexual advances.
In contrast to the more lurid allegations, the most important charges againstthe brothers will be mundane by comparison. are accused of issuing orders for the crackdown that led to the 2011 uprising against their father's rule.
In Libya's uncertain political environment, no one can predict whether the trial will be fair.
The International Criminal Court has issued a warrant for Saif, though not Saadi.