Johannes Jacobus Steyn, dubbed the "Sunday Rapist", had admitted to a kidnapping, his ex-wife testified on Monday.
"He phoned me and said he kidnapped another girl. He [said he] is sorry and that he is a monster," she told the High Court sitting in the Palm Ridge Magistrate's Court in Alberton.
The court ordered that the woman not be identified.
Steyn is being tried for the murder of schoolgirls Louise de Waal and Lazanne Farmer.
He is also charged with 11 counts of rape, 10 of sexual assault, 10 of kidnapping, one of attempted sexual assault, one of attempted kidnapping, and two of assault.
He has pleaded not guilty to all the charges.
His ex-wife testified that he phoned her on 12 October 2010, the day of De Waal's murder, and told her he had kidnapped a girl. He also told her he was confused and he was going to leave. He said there would be police at his house and that she should tell them he was guilty.
Asked about his whereabouts on Sundays, she said he did community service at a hospital in the mornings for almost four hours for a previous conviction.
"It's what he told me."
Throughout his ex-wife's testimony, Steyn cried. He put his head in his hands, sighed, and constantly wiped his eyes.
Asked why he always brought a Bible into court with him, Steyn said: "It gives me courage".
He said his brother gave him the Bible in 2007. Steyn said Psalm 43 gave him courage. He kept a bookmark with pictures of his wife and child on that page.
The 11 girls Steyn allegedly attacked between 2008 and 2011 were aged between 11 and 18.