Two weeks after the 14-year-old confessed to her mother she thought about killing people "all the time", the girl killed 10-year-old Biddy Porter at a property in rural NSW.
Biddy, remembered as a vibrant, clever and creative child, died of multiple injuries while staying at the property in the school holidays on July 8, 2020.
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 and is overseen by the Mental Health Review Tribunal.
Two psychiatrists found the teenager had schizophrenia and was in acute psychosis at the time of the killing.
The teenager likely had delusions and a "loss of contact with reality", counsel assisting Peggy Dwyer SC told the inquest at Lidcombe Coroners Court.
Dr Dwyer also outlined the teenager's worsening mental health before the killing and her "disturbing" descent into psychosis, including acts of self-harm, an interest in knives and a sense of not feeling real.
After the teenager slaughtered six chickens on the family property in June 2019, her parents removed her bedroom door and snapped a video game in half.
When asked why she did not seek mental health support for her daughter instead, the teenager's mother said it did not occur to her.
"(The chickens) were to be culled so that was what was happening to them anyway," the mother told the inquest on Wednesday morning.
"Mental health was never on my radar.
"I never believed in mental health, I didn't understand mental health so it didn't come up to me as something I needed to do."
The mother insisted she was only upset about the teenager killing the chickens because she did not have permission to slaughter the hens, which had stopped laying eggs.
"I don't understand why that was such a big red flag," the woman told the inquest.
The inquest was earlier told the mother asked a naturopathic herbalist about her daughter's behaviour and the practitioner told her to call a GP.
The mother made an appointment with a doctor after the girl revealed she had intrusive thoughts about killing people, including her parents and sibling.
The doctor wrote a referral for a psychiatrist in Sydney, which the mother found in her email spam folder only after the first day of the inquest.
Exactly a week after the GP appointment, the teenager killed Biddy while the two were left alone in the farm house.
The inquest, which is examining possible systemic failings in the lead-up to Biddy's killing and any early warning signs, continues before NSW State Coroner Teresa O'Sullivan.
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