The case has mobilised defenders of women against male violence, with hundreds of thousands of people signing a petition for her release.
"I had to put an end to it," Valerie Bacot, 40, wrote in her book called "Everybody Knew", adding: "I was afraid, all the time."
Bacot was 12 when her mother's partner, Daniel Polette, who was 25 years her senior, raped her for the first time.
He was sent to prison, but after his release returned and resumed the serial rapes.
At 17, Bacot became pregnant, got thrown out of the house by her alcoholic mother, and went to live with Polette.
Polette, also a heavy drinker, became increasingly violent, attacking her with a hammer at one point.
He later ordered her to work as a prostitute for truck drivers passing through, using a Peugeot people carrier.
Investigators established that Polette threatened to kill her if she refused, pointing a gun at her many times.
When Polette started questioning her 14-year old daughter Karline about her budding sexuality, Bacot said she decided that "this has to stop".
On March 2016, after Polette ordered his wife to undergo yet another sexual humiliation by a client, she used the pistol that he kept hidden in the car to kill him with a single bullet to the back of the neck.
Bacot hid the body with the help of two of her four children. In October 2017 she was arrested, confessed, and one year later released pending her trial while remaining under judicial control.
Her lawyers said "the extreme violence that she suffered for 25 years and the fear that her daughter would be next" pushed her to kill Polette.
The same lawyers, Janine Bonaggiunta and Nathalie Tomasini, had already defended Jacqueline Sauvage, a French woman who was sentenced to 10 years in prison for killing her abusive husband but won a presidential pardon in 2016 after becoming a symbol for the fight against violence directed at women.
"These women who are victims of violence have no protection. The judiciary is still too slow, not reactive enough and too lenient towards the perpetrators who can continue to exercise their violent power," Bonaggiunta told AFP.
"This is precisely what can push a desperate woman to kill in order to survive," she said.
Bacot was "certain that she needed to commit this act to protect her children", a court evaluation found.
More than 500,000 people have signed a petition demanding that Bacot, who risks life in prison for murder, be cleared of the charge.
The trial in Chalon-sur-Saone, in France's central Burgundy region, is due to run through Friday.