Days before a 14-year-old killed her younger cousin she wrote "I've only mutilated small animals so far but just imagine how nice its gonna feel when I do it to a human.
"I had a really good dream on the fourth about killing someone and I just got some encouragement from that dream about what I'm going to do."
The now 15-year-old girl pleaded not guilty to murdering her 10-year-old cousin at a country NSW property in Gunnedah on July 8, 2020.
After a judge-alone trial in the NSW Supreme Court, Acting Justice Carolyn Simpson on Tuesday found "the act proved" but said the girl was not criminally responsible due to mental illness.
The girls cannot be named because of their ages, while details of the fatal injuries are subject to a non-publication order.
Psychiatrists concluded that, at the time, the girl was suffering from untreated schizophrenia.
Her condition included auditory hallucinations coaxing her to kill and delusions her life and experiences were not real.
In continuing the suppression order on the girl's injuries, Justice Simpson said their publication would endanger the already precarious mental health of her family.
The teenager's July 6 diary entry included "I'm getting sick and tired of being told I'm a liar when I say i see cat eyes in a dark room and feel like I'm going to kill someone".
She wrote a plan for "my serial murders" starting with descriptions of what she would do to her family.
According to the agreed statement of facts, in the previous year she revealed she had killed six of her mother's chickens but did not say why.
She took parts of a chicken carcass to school, showing her friends how she killed it, and seemed to be "excited or proud" about it.
She was fascinated with knives, would prick or stab her friends with pens, and self-harmed.
Other students sometimes called her "psycho bitch" and about a month before the killing she told a friend about a voice in her mind that told her to hurt people, but she didn't want to.
She told friends she loved satan and had a demon friend under the bed.
Two weeks before the killing she told her mother she thought about killing people all the time, leading to a teleconference meeting with a psychiatrist.
She was referred to a Sydney doctor but the appointment was after the killing.
Six victim impact statements were presented to the court.
The 10-year-old's mother said she was unable to block out the guilt she felt in being unable to hold her daughter as she died, alone in a terrible place.
"No one teaches you how to cope with this type of loss, this type of pain," she said.
Her brain tried to shut out the events which were "so depraved they belong in a horror movie".
Her daughter had been a free-spirit and full of curiosity.
The victim's paternal grandmother told the judge of her pain over the "callous, calculated and morally reprehensible" slaying of her granddaughter.
"I will never speak the defendant's name - she doesn't deserve a name," she said.
"We are haunted by the terror, pain and incomprehension that (the girl) must have gone through to die that way. Terrified, alone, helpless and innocent."
She was caught totally by surprise in a place that should have been safe.
The young girl was full of promise, clever, cheeky, loving, intelligent and sassy.
"She didn't even get to finish year 5," the grandmother said.
The teenager will remain in detention under supervision and will be reviewed by the Mental Health Tribunal until deemed fit for release.