"When I learned of this case I was overcome by the horror of what happened, imagining the smell of blood, this butchery in the bathroom, this violence," Julien Eyraud told the court.
The woman, 34-year-old Hui Zhang, has admitted killing the couple, Ying Wang and Liangsi Xui, during a violent row at her home after they learned that their two-month-old baby had died accidentally while in her care.
"It's true, I killed them, and I will regret it for the rest of my life," the diminutive defendant said at the start of the trial on Tuesday.
Hui said she was acting in self-defence as the furious parents of the dead baby attacked her and her boyfriend Te Lu with a butcher's knife.
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She and Te had decided to offer the child's parents money to try to get them not to report the infant boy's death.
They invited the parents to their home, but said their plans quickly went awry faced with the fury of the grieving couple.
"I was sucked into a whirlwind of nightmares but I am innocent," Te told the court.
Hui then wrapped the body parts in rubbish bags and scrubbed her apartment clean.
Te later helped her get rid of the remains, transporting them "by foot or public transport" to the forest of Vincennes east of Paris, a policeman said.
Eyraud cast doubt on Hui's claim of self-defence and said there was no proof Te had taken part in the murders.
Several days later, a guide dog found a human torso in the same area, but the hunt for further remains was fruitless.
Before the bodies could be identified, Hui and Te turned themselves in.
They then directed police to the locations of more body parts around the forest.