A Perth man accused of fatally stabbing his mother, eight-year-old brother and 15-year-old sister claimed to be "greater than Jesus" and that serial killers were chasing him while detained in hospital days before the slayings.
Teancum Vernon Petersen-Crofts, 21, is facing a judge-alone trial in the WA Supreme Court over the deaths of his mother Michelle Petersen, 48, eight-year-old brother Rua and 15-year-old sister Bella at their Ellenbrook home on July 15, 2018.
He has pleaded not guilty on unsoundness of mind to murdering the trio.
The court on Friday heard harrowing details about the killings and Petersen-Crofts' increasingly bizarre behaviour in the preceding days.
Prosecutor Paul Usher told the court each of the victims had received between 44 and 54 stab wounds, while neighbours had reported hearing screams and what one person described as sounding like a "growling animal".
He said Bella was still alive in the backyard when police arrived and that paramedics had reported her softly pleading for help and asking if she was going to die.
She died in an ambulance on the way to hospital.
The court was shown a photo of the alleged murder weapon, a large kitchen knife with a 20cm blade which was covered in blood.
CCTV footage of a shirtless Petersen-Crofts pacing outside a nearby 7-Eleven service station early on July 15 and then speaking to an employee through a service window was also played, as was a recording of a triple-zero call made by the worker.
Mr Usher said Petersen-Crofts, who had blood on his hands, told the employee he had just killed his mother and his siblings.
"If I didn't kill them, they would have killed me," he told the worker.
"I killed them because they were hitting me with a club and poking me with a knife. My siblings were laughing at me."
The court heard Ms Petersen had told friends she feared for herself and her younger children because of Petersen-Crofts' behaviour.
Mr Usher said Petersen-Crofts, who had a history of mental health issues, had admitted intravenous methamphetamine use in the weeks leading up to the killings.
He turned up at his local police station on July 13, shirtless and sweating profusely and claiming his neighbour was a serial killer.
Petersen-Crofts was detained and transferred to St John of God Hospital in Midland where doctors said he was likely floridly psychotic.
But the following day he appeared calm and doctors decided to discharge him, despite his mother expressing concerns, Mr Usher said.
He attended Rua's baptism that afternoon and was described as talking about gods and goddesses and life after death.
The court was also shown footage of a police interview conducted two weeks before the slayings when Petersen-Crofts was arrested on suspicion of breaching a protective bail order related to his mother, who he described as "evil".
"Why is my mum telling lies when I've only shown her niceness? Can't she be arrested for this?" he asked the officers.
"I know what she's doing. She's getting in my head. She just wants to get payback."
Petersen-Crofts has been detained in a secure psychiatric facility since his arrest.
A senior homicide squad officer said at the time of the killings that it was "potentially the most horrific crime scene" he had seen.
