A teen charged with the murder of a family friend whom he believed to be a serial killer that had taken the souls of his friends has been deemed not guilty by reason of insanity.

Thanakorn Wittayanil stabbed David Dutiyabodhi 15 times in April 2021 under the delusion the victim was a serial killer who was controlled by an evil God and deserved to die.

Prior to his death in April last year, Dutiyabodhi had recently bought a house on Aileen Pl, Nawton in Hamilton where he lived with his son.

On the night of April 27, the victim's son invited Wittayanil, who was known Nicky, and his brother over to visit the following day. The 18-year-old said his brother was busy but he would visit.

He turned up at 10am and the pair went into a bedroom where Wittayanil began acting strangely, sitting on the edge of his bed with his head on his knees. He then left the bedroom, and went to the kitchen before going back upstairs and asking where his friend's father was.

He was directed to his bedroom, where Dutiyabodhi was lying on his back on the bed.

Wittayanil then got on top of him and stabbed him in the neck and upper body 15 times.

The victim cried for help while his son ran to help and Wittayanil left the room, dropping the knife.

The victim's son asked him why he stabbed his father to which Wittayanil responded, "I am God".

"I killed him because I had to. I am evil and it doesn't matter."

He then asked the son to go with him to Cambridge to which the son refused and instead tried to help his father and called 111.

Wittayanil used the bathroom sink before leaving the house.

He was later arrested at his home in Cambridge.

Crown prosecutor Jacinda Hamilton said the expert evidence was overwhelming that Wittayanil was insane at the time of the killing.

"The doctors came to effectively the same conclusion and it's an overwhelming one."

Wittayanil will now be detained at a mental health facility to receive treatment as a special patient.

His counsel, Mike McIvor, agreed with the psychiatrist's findings and said there was no medical evidence to support a permanent name suppression application.

He also acknowledged families of both the victim and Wittayanil were in court.

Police arrested Thanakorn Wittayanil at his home in Jasmine Pl, Cambridge, a couple of hours after the murder. Photo / Belinda Feek

Justice Mark Woolford said he was also satisfied that Wittayanil should be found not guilty by reason of insanity.

However, he said it wasn't simply a case of "rubber stamping" the medical professionals' findings.

Although he was acutely unwell when he was arrested he wasn't interviewed until he had recovered and had admitted killing the victim.

He noted that Wittayanil was suffering from acute symptoms of schizophrenia - recognised as a disease of the mind - at the time of the offending.

"He was aware that he was killing a person and that Mr Dutiyabodhi was the person he wanted to kill. Mr Wittayanil appeared to understand the nature and quality of his actions.

"He was aware the victim would die and the intention was to kill him."

However, due to suffering from that disease of mind, it meant Wittayanil didn't know killing him was morally wrong.

Instead, after police inspected phones of his friends, he was seen discussing bizarre religious delusions about being a God and his relationship to Thanos, the evil character in the Avengers movie series, prior to the killing.

"He went to the victim's home specifically to stab him because he'd had messages to kill the victim. He believed the victim was a serial killer that was controlled by an evil God, Thanos, and deserved to die.

"He was given a task to kill the victim related to his belief that he was a human God and was on a trajectory to become a full God.

"He believed he had killed several of his friends previously ... although they were still alive, he didn't believe he had actually killed the victims, he believed the victims' souls were no longer in the body."

Wittayanil had also twice previously been treated for mental health issues in 2020, when expressing delusions about his family and religious delusions.

He was discharged in December 2020 and then declined further offers of support.

- Belinda Feek, Open Justice