Hosea Joel Gerrard-Collier thought he was possessed by the Devil when he killed his late father's pregnant partner Robyn Alicia Crawford.

A young man who killed his father's pregnant partner suffered from hallucinations believed he was possessed by the devil and the world was coming to an end.

Robyn Alicia Crawford bled to death inside her Opotiki home in February in an unprovoked attack in which her neck was struck with a blade several times.

After the brutal death, her killer decapitated a sheep and carried the head around.

She was eight months pregnant at the time. The father of the unborn baby was her partner, Peter Collier, who died in December.

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His son, Hosea Joel Gerrard-Collier, was charged with the murder of Ms Crawford but was this week found not guilty by reason of insanity.

Justice Graham Lang was satisfied the 27-year-old was suffering a disease of the mind at the time he killed his father's partner, with whom he had a good relationship.

The evidence of his insanity was "compelling", according to Justice Lang, who read five reports from four different psychiatrists.

All four of the experts agreed Gerrard-Collier had suffered from schizophrenia for quite some time.

A motor vehicle accident in 2011 in which Gerrard-Collier suffered brain damage from being thrown against the windscreen, as well as drinking alcohol to excess, are believed to be contributing factors to his schizophrenia.

His mental health began to deteriorate significantly in February 2017 and he was seen behaving oddly, talking to people who were not there.

"He was paranoid and fearful of unknown persons," said Justice Lang. "He also began hearing voices."

Gerrard-Collier came to the attention of mental health services on several occasions but, at least twice, he was placed under treatment. On both occasions, he left before the treatment had either started or finished.

His state of mind deteriorated further after serving a prison sentence in 2017, from which he was released one month before killing Ms Crawford.

Before being sent to prison, Gerrard-Collier's mother said he knew he was unwell.

"On his release, however, he had no insight at all into his mental state," said Justice Lang.

His personality had changed, according to his mother; he was "remote" and virtually silent in the days leading up to the death of Ms Crawford.

From reading the reports of the psychiatrists, Justice Lang was persuaded Gerrard-Collier knew his actions were killing Ms Crawford and intended to do so.

However, the experts were unanimous in their opinion that Gerrard-Collier was incapable of knowing what he did was morally wrong.

"He was overwhelmed with a belief he was marked by the devil, was receiving commands from the spiritual world, the world was coming to an end and his body was outside of his control," wrote Dr Peter Dean in his report.

"He believed he was going to be tortured in hell for eternity if he did not follow the commands he was given."

Justice Lang was persuaded "beyond any doubt" Gerrard-Collier was insane at the time of killing Ms Crawford.

He will be detained in the Henry Rongomau Bennett Centre in Hamilton until further order of the court.