Schoolgirl Katie Rough was suffocated and had her throat slashed with a Stanley knife on a playing field in York in January, the BBC reports. She was also found with a 20cm cut to her torso.
A Crown Court has heard that the teenager, who cannot be named for legal reasons, was aged 15 at the time and asphyxiated the little girl while suffering "macabre" delusions. She has pleaded guilty to manslaughter by diminished responsibility.
The girl allegedly left a note beside Katie that said: "They are not human, they are not real".
The Sun reports police had found a stuffed toy from The Lion King in the unnamed girl’s bedroom that had its chest cut open and ears removed.
Graham Reeds QC, prosecuting, told the court the teenager had severe mental health problems, which led to her being removed from school in 2016. He explained the girl had a history of depression, had been self-harming for months and had been experiencing delusions that people "may not be human and may be controlled by a higher and hostile force".
News.com.au reports the court heard the defendant had been affected by a schizophrenic-personality disorder when she confronted the little girl.