The adultery of his ex-wife Ina Bonnette was treasonous, the so-called "Modimolle monster" Johan Kotze told the High Court in Pretoria on Thursday.
"I do not get renounced like that. It was treason against me," Kotze said.
He was responding to a question by prosecutor Retha Meintjes.
She asked about his emotions on the night he found out about his then wife's infidelity. He went to a dance on New Year's Eve 2011 and saw her lovingly interact with another man.
Kotze testified he watched them without their knowledge.
"I could not believe what I saw."
Kotze said however difficult it was for him, he still went to her home and watched them further.
"It just broadened the picture. It was a total lie."
Kotze said he had many questions about her fidelity during their marriage and observing what he saw on New Year's Eve explained a lot to him.
"It is unacceptable."
Meintjes asked what more he hoped to see by following them to Bonnette's house. She argued he already saw she was with someone else.
"Exactly what I saw that night... I'm happy I saw it," Kotze said.
He testified he saw them kissing and the man touching her breasts.
Meintjes told him despite watching them he never acted or confronted them.
"I am not someone who would storm in and hurt people," Kotze said.
He said he was betrayed by her infidelity. He had sold his farm in Bloemfontein and they had bought a house together.
"We were supposed to become one family."
Kotze is accused of orchestrating the gang rape of Bonnette, and of murdering his stepson Conrad, 19.
His co-accused Andries Sithole, Pieta Mohlane, and Frans Mphaka are also charged with kidnapping, assaulting, repeatedly raping, and attempting to murder Bonnette that day.
Bonnette and Kotze's daughter Jo-Marie Kotze sat in the public gallery again on Thursday.
Earlier Kotze told the court he could not remember mutilating Bonnette's breasts and genitals.
"Your Honour, these things happened... but I can't remember what happened that day," he said.
Meintjes described Bonnette's injuries to Kotze. He either denied inflicting them or said he could not remember doing so.
Kotze also denied shooting Conrad intentionally.
He told the court he remembered that Conrad had said: "No uncle, don't."
"Everything just happened. I just heard the shot."
Kotze said he had been surprised to learn Conrad had been shot three times, and maintained he did not aim at Conrad.
"I can't remember something like that at all... There was no reason. I did not want to do it," he said.
At the end of the day Judge Bert Bam asked Kotze if he thought about what happened when he left the house, if he thought he had committed an offence.
"Not at all," Kotze said.
"I did not think of Ina or [Conrad] when I left the house."
Bam asked why he then went on the run. Kotze explained he did not flee, but merely got into his car and drove.
The trial continues.