JOHANNESBURG, MAY 25 -- The leader of Winnie Mandela's former retinue of young bodyguards was convicted today of murdering a 14-year-old anti-apartheid activist last year following beatings in which the wife of African National Congress leader Nelson Mandela was implicated but not charged.

Judge Brian O'Donovan also found Jerry Richardson, 41, guilty of abducting the victim and three other young men from a Methodist halfway house in Soweto on Dec. 29, 1988, and beating them at Winnie Mandela's house.

Richardson, coach of the Mandela United Football Club, whose members doubled as Winnie Mandela's bodyguards, had "played a leading role" in the subsequent death of Stompie Seipei, the district judge ruled in Johannesburg. He set sentencing for August.

Prosectors said they are still considering whether to charge Mandela. Witnesses, including the three men abducted, testified during the three-week trial that she had interrogated and beaten them. However, the witnesses said she was not present when the most severe beatings occurred.

The body of Seipei, whom Richardson and Mandela had accused of being a police informer, was found in a vacant lot in Soweto, near Johannesburg, in mid-January 1989. An autopsy showed that he had been severely beaten and that three stab wounds to the neck caused his death.

Shortly after the incident, Mandela, known as the "Mother of the Nation" who fought police harassment, banning orders and detention most of her life, was publicly censured by anti-apartheid leaders.

Since Nelson Mandela's Feb. 11 release from prison, Winnie Mandela has kept a low political profile. Out of respect for her husband's stature, leaders of the movement opposing the government's apartheid policy of racial separation have begun to repair their rift with her.

Nelson Mandela complained this week that the government was unfairly tarnishing his wife's reputation. "When my wife is not charged and the whole case centers around her, she has no way of defending herself and her innocence," he told reporters. Winnie Mandela, acting on her lawyers' advice, had declined to testify in Richardson's trial because of the possibility that she might later be charged.

One of the abducted men, Kenny Kgase, 31, testified that Mandela entered the room where they were held in her house and began angrily questioning them. Then, Kgase said, she asked for a sjambok, a short whip, and delivered a few blows to both Kgase and Seipei.

On Jan. 1, 1989, Richardson gathered Seipei's belongings and the young man was taken from the house. Richardson returned the next day alone, with what appeared to be blood on his shoes, Kgase said.