Cecilia Steyn was the "mastermind" behind the Krugersdorp murders, the South Gauteng High Court has found.

Steyn, 37 - along with Marcel Steyn, 21, and Zak Valentine, 33 - pleaded not guilty to 32 counts, including murder, robbery with aggravating circumstances, fraud and intimidation.

Marcel's brother Le Roux Steyn entered into a plea bargain with the state in 2018.

Marcel's mother Marinda Steyn, 51, has already been sentenced to 11 life terms for the killing spree.

They were all part of Electus Per Deus, a group that viewed themselves as chosen by God. The group killed 11 people between 2012 and 2016.

"Each of the accused directly or indirectly took part ... with Cecilia being the mastermind and Marinda being second-in-command," said judge Ellem Francis as he delivered judgment on Monday.

Francis said it was Cecilia and/or Marinda who had issued instructions to other members of the "enterprise", while Zak and Marcel were foot soldiers. 

Cecilia was part of the planning of the offences and was the beneficiary of the proceeds, the court found. "Cecilia benefited from all proceeds and made sure she did not dirty her hands. She was at all times briefed of all the criminal activities," said the judge.

"I'm satisfied that the work the Electus Per Deus were doing fits neatly into the work of an enterprise," he said, adding that he was also satisfied that Marinda and Cecilia managed the affairs of the group.

The court noted that Marcel's evidence corroborated what witnesses had told the court - that meetings at which the murders were plotted were held at Cecilia's flat.

Dealing with each count faced by the three accused, Francis said he was satisfied that the state had proved a case of racketeering against them. "There was a pattern of racketeering. There was overwhelming evidence to prove this," he said.

He said the state had also proved that the trio was involved in bombings and the manufacturing of explosives. "All accused were involved or acted in common purpose. They were either involved as perpetrators or accessories," ruled Francis.

According to the court, Marcel's change of heart to tell the truth remained a challenge as her evidence could not be tested.

Francis said the murders of people who were close to Ria Groenewald were not random acts of violence but were intended to cause her harm. "The deceased were emotionally involved [with] Ria Groenewald," he said.