A man stabbed his four-year-old son to death over fears the boy would become a psychopath like him, court documents have revealed.

David Janzow killed his son Luca in Adelaide in July last year, with a court now finding him not guilty of murder by reason of mental incompetence.

Documents tendered to the court said Janzow was diagnosed with bipolar disorder and had a long history of depression.

He told emergency service workers and psychiatrists that no-one deserved him and he was a psychopath.

They detailed Janzow's belief that his four-year-old son was like him, and he wanted to "save Luca from what was to come".

The documents said Janzow pulled up at a secluded location with a nice view and stabbed his son ... so that it would be over quickly.

He then tried to stab himself in the neck, before driving to a police patrol and alerting the officers to what he had done.

"I knew this was bad, but everyone would be better off. I knew murder was bad but I just had to get rid of us because there was no good bone in a psychopath," he told a psychiatrist.

"I discovered that I was a psychopath and that [Luca] had the same qualities in him. At two, Luca had a night terror and screamed for five hours — I had night terrors and screaming fits as a child. No-one in society wants a psychopath and it is characteristic to take their own or others' lives and I had those thoughts in the last week."

The documents revealed he told a police officer: "I need help, I've murdered my son" and later told a paramedic: "I'm a psychopath with mental health problems. I stabbed my son with a Leatherman [hunting knife]".

The boy died in hospital after Janzow alerted the police patrol at Athelstone in the Adelaide foothills that he had injured his son, who was in his ute.

The prosecution conceded Janzow was mentally incompetent at the time of the killing.

After receiving a series of psychiatric reports, Justice John Sulan found Janzow not guilty by reason of mental incompetence, but found the objective elements of the crime proved.

Janzow, who has been detained in a secure psychiatric facility, will now face a lifetime supervision order.

'I will grieve for Luca until my last breath'

A victim impact statement from the boy's mother, Julia Trinne, was read to the court in front of an emotional packed public gallery of her family and her husband's family.

"I will grieve for Luca until my last breath," she said in the statement.

"[He was] A beautiful-natured, kind, gentle, caring, happy, shy, tall, clever, cheeky, creative boy with the most incredible blue eyes, infectious smile and laugh.

"At the young age of four Luca had inner wisdom of someone far beyond his years."

She described the moment the doctor told her there was nothing they could do to save her son and they were preparing his body for her to see.

"Shattered, utterly and incomprehensible broken," she said.

"I wanted to get out of this world, I wanted to find Luca. It's a sadness I did not know was possible.

"I felt broken and I was broken. I wish Dave had taken me instead.

"Every part of the life I knew and lived no longer existed. The man who Luca and I loved so dearly was the man who took my son's life.

"I have lost my beautiful son Luca and I have lost my husband.

"The loss of Luca and the grief itself is devastating beyond comprehension, I felt at times I could've stopped breathing and sometimes I wish I could.

"I know deep inside I will never be the same."

Janzow cried in the dock as the victim impact statement was read to the court.

Family and supporters also cried throughout the hearing.

Mother wants tragedy to raise awareness of mental illness

Ms Trinne said in her statement to the court she wanted her incomprehensible tragedy to alert others to the need to support those who suffer from mental illness.

"In time, my hope is that this tragedy will bring awareness, understanding, support and promotion for mental health — and bring attention to the struggles that people with mental health issues and their supporters face," she said.

"I hope that the shocking devastation to our beautiful family of four can give other families additional reason to love and treasure each other more and to live an extra special life that Luca will not have and I will not have as the family we were."

She said she had loved her life with her husband and two sons.

David Janzow with son Luca, and the boy's mother Julia Trinne. ( Supplied )

"My relationship with Dave was special. We were a great team with many happy times," she said.

"Dave was a beautiful husband and when our boys arrived he was a loving and kind dad. He had been a gentle, supportive, encouraging, adventurous, entrepreneurial and motivated man.

"I loved sharing my life with Dave and I envisaged spending our lives together."

She said life had its challenges at that time.

"Life with Dave was mostly fun, adventurous and loving but it was also chaotic and it came with its ups and downs along the way," she said.

"Loving Dave was easy, that is not to say that life was easy because it certainly had its challenges."

She said there were challenges with her husband's mental health over the years including depression, but the escalation in his mental illness came suddenly and without warning.

"Dave became acutely unwell in a short period of time with absolutely tragic consequences," she said.

"There was no warning. There was no suggestion any of us were in danger.

"I loved Dave right up until this terrible tragedy. In most other circumstances I would've been by his side no matter what, I would've worn my wedding ring with pride until my last days but now I cannot.

"A living nightmare does not come close to what I have been going through.

"In time my hope is that this tragedy will bring understanding and awareness to mental health.

"Greater awareness and understanding of what professionals, families, friends and the community can do to make a positive difference to people with mental illness.

"There will be positive things to come out of this tragedy and I will make sure this happens."

She said she would always honour her son, and would forever miss the family life she cherished.

"The life we know and loved does not exist any more. I miss my son and husband and my family of four. "I miss them so much and have so much sadness. Sadness also for the many people whose hearts were also shattered that morning and for Dave also with what he now must live with. "My beautiful, beautiful son Luca — I have so many wishes. I wish I knew Daddy was so unwell. I wish I'd heard something that morning or that I got out of bed just a bit earlier than usual. I wish I had called triple-0 when I found out that you were both missing from our home. I wish I had been there for you to try to snap Dave's mind out of it — to stand in between you, or at the very least to have held you in my arms as you left this world. "I will forever honour Luca and in time I will create awareness around this tragedy and work towards living my life to the fullest. "I will always try to focus on the rainbows after the storms and I'd like to encourage other people to do the same."

The case returns to court in February when the conditions of Janzow's mental health licence are expected to be set.