Hours before the 14-year-old killed Biddy Porter on a NSW farm early on July 8, 2020, she texted a friend to say she "felt crazy" and was thinking about killing someone.
"It doesn't matter how many times I tell mum or dad I want to kill someone ... they think I'm joking," the distressed teenager said in a text to a friend late on July 7.
In the year leading up to the killing, the teenager slaughtered six chickens on the property, reported seeing the eyes of the Cheshire cat in the dark and repeatedly told relatives she "didn't feel real".
She also had a collection of knives in her room, each of which she named.
Those were among the signs of her worsening mental state and emerging psychosis, a long-awaited inquest into Biddy's death has been told.
The NSW Supreme Court found Biddy's killer, who cannot be identified for legal reasons, was not criminally responsible due to mental illness in 2021.
She has been in custody since her arrest.
Two psychiatrists found the teenager had schizophrenia and was in acute psychosis at the time of the killing, which happened when Biddy was staying on the farm during school holidays.
That mental illness meant the teenager likely had a "loss of contact with reality", counsel assisting Peggy Dwyer SC told Lidcombe Coroners Court on Tuesday.
On July 7, the teenager suggested playing "serial killers" to Biddy and another child, Dr Dwyer said.
"That was so disturbing the children were frightened," she said.
The four-day inquest will examine possible systemic failings in the lead-up to Biddy's killing, along with any early warning signs.
Psychiatrist Olav Nielssen is expected to tell the inquest there were missed opportunities to help the teenager before she killed Biddy, Dr Dwyer said.
The teenager's father told the inquest he thought her behaviour was down to a combination of school bullying after COVID-19 lockdowns, playing video games and watching inappropriate movies.
He was not enthusiastic about his wife going to a GP for a referral to a psychiatrist when the teenager reported hearing voices and having intrusive thoughts soon before the killing.
"It could've been a missed opportunity, at that time I didn't understand," the father told the inquest.
"Bullying, that's where my head was at the whole time."
Biddy's parents, Rebekah Keukenmeester and Dominic Porter, campaigned for the inquest and a separate inquiry in the hopes no other family have to endure a similar tragedy.
Dr Dwyer said Biddy was a "vibrant, beautiful, clever child", who was deeply loved and known for her creativity, kindness and sense of justice.
There were no sufficient words to describe Biddy's parents' grief and trauma, Dr Dwyer said.
The coroners court was not an arena for blame or shame, but rather to find out what happened to Biddy so others can learn, she said.
"It's not to blame, but to help teach counsellors, GPs, parents and grandparents.
"If you notice signs and symptoms like this, how to you distinguish them from other teenage behaviour and how do you get help?"
The inquest continues before NSW State Coroner Teresa O'Sullivan.
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