ST-JEROME, Que. - Isabelle Gaston looked directly at her estranged husband in a Quebec courtroom Thursday and told him he wasn't a bad parent -- even if he ended up killing their two young children.

Testifying at the trial of Quebec cardiologist Guy Turcotte, Gaston turned to her ex in the prisoner's box.

"You weren't a bad father, that I know," she said sobbing. "And I'll never say you were a bad father. Never."

It was one of several emotionally charged moments in the courtroom Thursday as a grieving mother shared loving memories of her two children killed by Turcotte, a once respected doctor.

About half the courtroom wept while listening to Gaston's testimony.

Even Turcotte, 39, began weeping when he heard his wife's name and those of his kids.

She testified that the couple always had its problems but she never believed Turcotte would hurt his own kids.

She is one of the final Crown witnesses in its case against Turcotte before the defence takes over.

Turcotte has pleaded not guilty to two counts of first-degree murder in the deaths of five-year-old Olivier and three-year-old Anne-Sophie in February 2009.

They were stabbed to death in a rented home in Piedmont, Que., where Turcotte was also discovered after having ingested washer fluid.

Turcotte admits to killing the children but has denied intent. The defence has yet to present its case.

The testimony was so highly anticipated that members of the public began lining up an hour-and-a-half before the courthouse opened, just to get a seat.

While she often broke down on the stand, Gaston plowed through nearly two hours of testimony, recalling the final moments with her children.

She dressed Olivier the morning of Feb. 20, 2009, and gave him a kiss as he went off to school. He waved back at her, the final time Gaston saw him.

A few hours later, Anne-Sophie would be dropped off at daycare. But before setting off, Gaston scurried around the car for her daily exchange of a minimum of 20 kisses with the toddler, who was sitting in the child seat.

"Do you know how much I love you?," Gascon says the little girl told her as they shared their smooches.

The jury also heard that tensions had flared for an umpteenth and final time between Gaston and Turcotte when she had the locks changed at the home the couple once shared.

She switched them following a number of run-ins between her ex and her new boyfriend, Martin Huot.

She testified that Turcotte was livid.

"You want a war? You've got a war," she recalled him saying over the phone.

Gaston panicked at the threat. But she felt the warning was related to money.

She ended up deciding to go on a girls' weekend northeast of Quebec City. The next day, the kids were found dead. She found out about it on the news.

"I feel stupid for not thinking that he'd hurt the kids," Gaston said. "I never thought for a second he'd do that."

Gaston, an emergency-room physician at Hotel Dieu Hospital in St-Jerome, also cried as she told the jury about her relationship with Turcotte, which began when they were medical students in 1999.

"From the beginning we had our highs and lows," Gaston said.

"In 10 years I couldn't say it was all black or all white."

The couple fought -- and fought a lot -- about everything from parenting and money to intimacy and kids' extracurriculars.

Gaston said she tried to improve their relationship through reading self-help books and employing a life coach.

Despite the good that came with Olivier's birth in 2003 and Anne-Sophie's in 2005, there was also plenty of bad.

Gaston said they separated once, early on while dating, after she found gay porn on his computer. She found more porn again in 2008.

Although she confronted him, Turcotte vehemently denied being gay.

That discovery, coupled with the arguments and a lack of intimacy in their marriage, would be the beginning of the end, she said.

Gaston began a new relationship with Huot, a personal trainer. It blossomed quickly but she decided to keep it secret.

"I regret today not having told Guy myself earlier," Gaston said, referring to how Turcotte heard the news from a third party -- Huot's ex.

"I waited because of Anne-Sophie and Olivier."

Gaston and Huot are still together today.

Gaston defended Turcotte as a good father while they were together. She said each contributed as a parent.

"Mothers and fathers don't always see eye to eye, but I think we were complementary," Gaston said.

Even when the couple split, she had hopes they might still be friends. She said she wanted them to be "a team" -- to work together on raising the kids.

She last spoke to Turcotte on May 17, 2009, when she called a psychiatric hospital in Montreal and an operator transferred the call.

Gaston hadn't gone to work that day and was thinking of committing suicide herself. She had already written the letter. First, she wanted to ask Turcotte: why?

"Why the kids, Guy? I loved them more than I love myself," she told him.

He replied, according to Gaston's testimony: "Me too."

The trial continues Friday.