WARNING: This story contains details some readers may find disturbing

Kellie Johnson was not criminally responsible when she killed her five-year-old son two years ago, her defence lawyer plans to argue.

The Saskatoon woman, who appeared in Queen’s Bench court Tuesday for day one of her trial, pleaded not guilty to first-degree murder in her son Jonathan Vetter’s death.

The boy was found dead Jan. 4, 2014, in a home on the 400 block of Avenue R South.

Both Crown and defence lawyers agree Johnson killed her son, but the defence is planning to argue — when the trial resumes in May — the 37-year-old’s mental state did not allow her to understand what she was doing was wrong.

“A defence that we are putting forward is that Miss Johnson, at the time that she took her son’s life, was not criminally responsible,” defence lawyer Leslie Sullivan said outside court Tuesday.

According to the agreed statement of facts in the case, Johnson bought a large kitchen knife in December 2013. Two weeks later, on an early morning the following month, she used the knife to slit her son's throat while he was sleeping. Johnson's other son awoke to his brother gasping.

Johnson said, “Oh my gosh, I'm so sorry,” then ran away.

She left the home, changed her bloody pants in a hospital bathroom and ate a meal at McDonald’s, according to the document. She then took a cab to Royal University Hospital and told a nurse she hallucinated and may have hurt her son. She was only trying to save him, she told the nurse.

She later told a police investigator, according to the agreed facts, she had a “bad hallucination.”

“I thought this lady was going to send my son to hell. You know the devil and do it by continually molesting him. I was thinking how to fix it I realized I had to send him to heaven before she could get to him.”

In court on Tuesday, a recorded police interview conducted the day Vetter died was played.

In the recording, Johnson talks about a woman — she described the woman as a “devil person” — who had been continually harassing her for about four years.

“I don’t know if you believe in the devil, but I believe she is the devil, like a devil person in this world,” Johnson said during the two-hour interview.

She mentioned the woman a number of times and said the woman threatens to harm her and her children. The hallucinations were so severe she tried getting help from various churches, she told the officer.

“They were showing me this kind of stuff, really scary stuff, and this lady and other people — it went on all night sometimes,” Johnson said in the interview. “I didn’t sleep for a year. It was that bad.”

She also told the officer she spent time at a mental health facility. She had taken antipsychotic medication off and on for about six years, she said.

On numerous occasions in the interview, Johnson was asked about the events leading up to Vetter’s death. She almost always replied by asking for a lawyer.

She also said, near the beginning of the interview, she understands what’s going on around her and is aware of what’s happening.

Her trial, which was set to begin Monday, was pushed to Tuesday because her defence lawyer was ill. The remainder of the trial was scheduled to continue this week, but because of another illness — this time to a key witness — the trial won’t resume until May.

Sullivan, Johnson’s defence lawyer, said the defence team is planning to call two doctors who are currently treating Johnson at the Saskatchewan Hospital in North Battleford to the stand.  

Crown prosecutor Brian Hendrickson is expected to call one rebuttal witness.

Johnson will remain in custody at Saskatchewan Hospital until the trial continues.

--- CTV’s Angelina Irinici was in court for day one of the trial. View her live coverage below: