At the North Gauteng High Court on Monday, Judge Bert Bam found him and one of his co-accused, Andries Sithole, guilty of killing Ina Bonnette's son.
The other two co-accused were acquitted on the murder charge.
Kotzé, Sithole, Pieta Mohlane and Sello Mphaka are on trial for the murder of Kotzé's stepson Conrad Bonnette (19), and kidnapping, repeatedly raping and attempting to murder Ina Bonnette in his Modimolle, Limpopo, home on January 3 2012.
However, Kotze, Sithole, Pieta Mohlake and Sello Mphaka were all convicted of kidnapping and of four charges relating to the rape of Bonnette.
"It is obvious that all four are guilty of it," said Bam.
In addition, Kotze was found guilty of assault with intent to do grievous bodily harm, instead of the more serious charge of attempted murder.
Afterwards, as Bonnette looked at the four men in the dock, a friend told her: "You are allowed to cry Ina Bonnette. You are allowed to cry."
Asked how she felt about the judgment, Bonnette said: "I am okay". She declined to give further comment.
Kotze hugged friends in the gallery before being taken down to the holding cells.
He has denied guilt, claiming he could not be held accountable for his actions that day. Sithole, Mohlane and Mphaka claimed Kotzé threatened to shoot them if they did not cooperate.
Earlier on Monday, Bam said Bonnette was cross-examined by three experienced attorneys. He said this must have been traumatic and painful for her.
"She is just a human being. Any person makes mistakes," Bam said.
Unsure at times
Bam said it was not uncommon for a witness, who was as emotional as Bonnette, to be unsure at times. He said she was not a perfect witness, and that it was not realistic to think such a person existed.
"There was a sound of truth [to her testimony]."
One of many examples of this was her testifying that, after the rape, she pleaded with her eyes for Kotzé's three co-accused to help her.
"It is unlikely that any witness would think of something like that," said Bam.
National Prosecuting Authority spokesperson Medupi Simasiku welcomed the judgment.
"Yes, we are happy and we hope it brings closure to the family." He hoped the judgment would serve as a lesson that these types of crimes did not go unpunished.
The matter was postponed to Tuesday morning for arguments in mitigation of sentencing. – Sapa