A Sydney man was harbouring delusional beliefs that his mental health case manager was a "hit man" when he stabbed the nurse to death.
Peter John Kemball, 41, was found not guilty on Wednesday of murder on the grounds of mental illness after killing Stephen James Douglas on November 28, 2019.
The 62-year-old mental health advocate and registered nurse was attacked by Kemball in his Balmain East unit after Mr Douglas told his co-worker to wait downstairs saying he did not feel "unsafe" with Kemball.
But Kemball had delusional beliefs about his case manager, previously telling his father Mr Douglas was a "hit man" and "I'm his job".
Kemball later told psychiatrists he believed Mr Douglas was Satan, a rapist and that he was acting in self-defence.
In the weeks leading up to the assault family and friends had noticed a deterioration in his mental health including delusional beliefs that Malcolm Turnbull and Julie Bishop were posing as his parents, conspiring to ruin his life.
Two days before he inflicted horrific injuries upon Mr Douglas he was given about 0.5ml less of the usual 1.9ml dose of his anti-psychotic medication Abilify, according to the facts of the case.
When the general practitioner realised the vial contained the remainder of the medication she booked Kemball in for a follow-up appointment but he never showed.
On the morning of the attack Kemball visited his parents' home where he hit his mother on the head three times with a cushion, shook a visiting neighbour and yelled at a nearby carpenter.
His father was so concerned with the behaviour he contacted Mr Douglas and requested an appointment with his son.
But not long after Mr Douglas arrived at his home, Kemball phoned his father and told him "I just killed my case manager".
"That's not funny. Don't joke about this. Where is he?" his father responded.
Kemball sent a photograph to his father who recognised a tattoo on Mr Douglas' body which had been placed underneath a sofa couch in the living room.
He dialled triple zero immediately as Mr Douglas' co-worker knocked on Kemball's apartment and called his mobile, and was told her colleague had "stabbed himself to death".
Justice Peter Johnson, who presided over the NSW Supreme Court trial without a jury, said the unanimous psychiatric evidence concluded Kemball was suffering from a longstanding "disease of the mind" of either schizoaffective disorder or schizophrenia.
He said Mr Douglas was performing an important public duty and his loss was a loss to the entire community of NSW.
Douglas, a well-loved and dedicated father and grandfather, had been so excited to meet his fourth grandchild before his life was brutally ended, the court heard.
Outside the court on Wednesday, his daughters-in-law said the family were finding it extremely difficult to come to terms with the fact he died while attempting to do the job he always performed with great care, compassion and respect.
"We are hopeful that a result of Stephen's death in these tragic circumstances... much-needed changes will occur to ensure mental health workers are adequately resourced and supported to serve our community," Sarah and Emma Douglas said in a statement.
Justice Johnson placed Kemball in the long-term care and control of the mental health tribunal which can only release him if it is satisfied he will not seriously endanger anyone including himself.
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