A St. Tammany Parish jury found 56-year-old Joan Faye Hartman guilty of manslaughter in the stabbing death of her son’s girlfriend and mother of her two granddaughters, reaching a verdict Friday night after just under two hours of deliberation.

The prosecution was seeking a verdict of second-degree murder, a more serious offense.

Sentencing will be next Friday, according to Rick Wood, a spokesman for District Attorney Warren Montgomery.

Hartman, who took the witness stand in her own defense Friday, told the jury in Judge Scott Gardner’s courtroom that she had one thought pounding in her head in July 2012 when she went to her bedroom to retrieve a knife she had bought a month earlier with the idea of killing Tanya Knower: H er contentious relationship with her son’s girlfriend had come down to the question of who would kill whom.

And, she said, she was determined that she was not the one who was going to die that night in their Slidell home.

Prosecutors worked to dismantle any claim of self-defense by the 56-year-old grandmother during cross-examination and closing arguments in the trial that began Tuesday.

Assistant District Attorney Jason Cuccia reminded the jury that Hartman, by her own admission, had stabbed the 27-year-old mother of two in the back. Knower had turned away from their confrontation when she saw the raised knife in Hartman’s hand, Cuccia said, and there should be no question who was the aggressor the night of July 15, 2012.

Assistant District Attorney Nick Noriea questioned the defendant closely about their fatal encounter, asking why the defendant had never sought help from law enforcement or social service agencies in her efforts to deal with her son’s family, which included two young daughters, one autistic.

“You decided it was better for the kids for you to kill Tanya than seek assistance,” Noriea said.

“It was better to kill Tanya than for her to kill me,” Hartman retorted heatedly.

“You get to be judge, jury and executioner,” Noreia said.

Hartman’s chilling jailhouse confession was played for the jury Thursday, and the defendant had watched impassively as she was heard angrily telling a detective that she had meant to kill Knower and was concerned only that she might have failed.

But as she again recounted that night on the stand Friday, Hartman grew agitated, her face flushed with anger and her voice loud.

She described fleeing the Slidell area home at 10 p.m. after a fight during which Knower denied her the keys to the car, and she said she walked along Gause Boulevard without the crutches she had been using for a sprained ankle. She returned about midnight, in part because she needed the diuretic she took for congestive heart failure.

But her attempt to cool off failed when she saw what Knower had done in her absence, she said. Meemaw’s Wall of Fame, which Hartman described as the place she posted pictures of her granddaughters and their artwork, had been stripped clean.

“She said, ‘My kids.’ I’m boiling ... now everybody is angry,” Hartman said, recalling she demanded the return of the pictures and then got into an altercation with her son, Richard Hartman, and Knower.

“She kept looking at me, saying, ‘You’ll never see the kids again,’ ” Hartman recounted, saying she told Knower, “Don’t go there.”

But the standoff escalated, culminating with Knower looking at her and saying a single word: “Dead.”

“She looked exactly like she looked eight years ago,” Hartman said, recounting the time Knower had stabbed her. The younger woman headed for the kitchen, and Hartman said she went to her bedroom and grabbed her knife.

Later in the afternoon, the prosecution showed her pictures of the trash cans at her home, which were filled with her medicines. But the defendant pointed out that her granddaughters’ drawings were also in the trash, something she said she had not known until that moment on the stand.

She denied that she had thrown away her drugs, saying she had no thoughts of suicide and would have flushed them down the drain had she wished to harm herself. She told the jury that her son frequently threw her drugs away when he was angry with her.

Hartman’s lawyer, Martin Regan, tried to shoot holes in the credibility of her son, Richard Hartman, who had testified earlier in the week.

Regan asked Joan Hartman if her son had lied when he said he had never choked or hit his mother. She said he had.

Regan then called a sheriff’s deputy from Oklahoma who testified about the arrest and conviction of Richard Hartman in 1997 for choking his mother and kicking her in the back.

The deputy, Terry Alexander, said Richard Hartman had said he had friends in the Aryan Nation who would kill his mother for getting him arrested.

The defense rested about 5 p.m. after a day of testimony that saw Hartman on the stand for about three hours.

Follow Sara Pagones on Twitter, @spagonesadvocat.