SAINT-JEROME - Guy Turcotte says he felt increasingly isolated and destroyed in the days leading up to their deaths.

The cardiologist accused of killing his two children said Wednesday that as his wife's new beau quickly started to become a fixture in her life and their children's, he felt more and more marginalized.

Turcotte is charged with two counts of first-degree murder in the 2009 stabbing deaths of the children less than a month after separating from his wife, Isabelle Gaston.

He has admitted to causing the deaths of Olivier, 5, and Anne-Sophie, 3, but has denied intent or premeditation.

The defence says the murder case should focus on Turcotte's state of mind at the time of the killings -- and not on whether he performed the act.

Turcotte's testimony of the final moments of the children's lives is expected Thursday.

He told his jury trial that he was sleeping poorly and crying a lot in the weeks leading up to the deaths but that he maintained a regular work schedule and signed papers to buy a new house.

Turcotte was on the stand for a third day in his own defence and repeatedly said he was discouraged, anxious, sad, destroyed, broken and angry following the split from Gaston.

A telling moment came when a lonely, miserable Turcotte spoke to his son on the phone on Feb. 8, 2009 -- less than two weeks before the children were killed.

Olivier recounted a fun-filled weekend at the Quebec City winter carnival hanging out with its beloved mascot, Bonhomme, as well as with his mother, sister and his mother's new man, Martin Huot.

"When he told me that I was paralyzed," Turcotte testified Wednesday.

"I wasn't able to say anything and I hung up. I was completely destroyed. I missed my kids. I saw Martin taking my place and it tore me apart."

He spoke of anger as a neighbour informed him that Huot had been at his former residence most nights since the couple had split.

"I freaked out," Turcotte said.

"I found out that a few hours after I left, that he's already sleeping in my bed, that he's taking my place, that he's there every night."

There was also a run-in with Huot at the family home on Feb. 10, where Turcotte punched him in the face.

Turcotte told the court he simply could not handle being replaced.

He recounted one particularly explosive argument after he'd barged into his former house to confront Huot a second time.

Gaston threatened to change the children's last names and said she could move anywhere she wanted in Quebec.

"It was like she wanted to exclude me from the kids' lives, dismiss me," Turcotte said.

"I was losing what I had left."

Turcotte admitted that after she told him she'd changed the locks and spoken to a lawyer, he told her:

"You want a war? You'll get it."

He explained what he meant was he'd get a lawyer and make sure Huot wouldn't be able to step foot in the family home.

The trial continues Thursday.