CCTV captured a "five- or six-minute" beating that left a man dead over a "drug deal gone wrong", a court has heard.
Gabriel Hikari Yad-Elohim is on trial for murder before a jury and Justice Gerard van Bohemen in the High Court at Auckland.
Michael David Mulholland's bloodied body was found in the stairwell at the Western Springs flats where he lived, on September 26 last year.
Yad-Elohim, 30, was arrested in central Auckland the following day.
In her opening address, Crown prosecutor Kirsten Lummis described the killing as a "drug deal gone wrong".
"[Yad-Elohim] was keen to get his hands on some drugs, his drugs of choice were methamphetamine and cannabis.
Lummis said it was Yad-Elohim's desire to obtain drugs which led to "the fatal events of that evening".
"The defendant beat Mr Mulholland to death."
However, the Japanese man and his lead defence counsel Annabel Cresswell are defending the charge and argue he is not guilty by reason of insanity.
"In fact the [Hebrew] name Gabriel Yad-Elohim is a name given to himself and it means messenger of God," Cresswell said.
Yad-Elohim, who has been remanded to the care of the Mason Clinic since he was charged, suffers from "treatment resistant" schizophrenia, Cresswell told the court.
The attack was sustained and brutal, Lummis said, with Mulholland suffering several kicks and stomps to his head.
Mulholland was a 69-year-old and weighed just 67kg - "no match for a younger fitter Mr Yad-Elohim", Lummis said.
The identity of his killer was never up for debate, Yad-Elohim killed Mulholland - CCTV filming the entire assault.
The footage, seen by the Herald, has been suppressed by Justice van Bohemen but shows Mulholland being "quickly rendered unconscious", Lummis said.
"The attack goes on for minutes, it is not brief, about five or six minutes," she added, also describing the blood splatter on the walls of the stairwell.
"It is somewhat of an unusual murder trial to have the entire incident captured on CCTV ... it's not pleasant.
"While the Crown can describe the [video], you need to see it for yourself," Lummis told the jury.
The prosecutor argued Yad-Elohim was motivated by having been ripped off in a drug deal, with Mulholland unable to give him the drugs he desired.
"A drug deal gone wrong, Mr Yad-Elohim had felt swindled, having lost $200," she said.
Lummis conceded Yad-Elohim suffered from mental health issues during the past 10 years and had been receiving treatment after voluntarily presenting himself to Auckland City Hospital in the days before Mulholland's death.
The Herald earlier revealed Yad-Elohim was a patient at Auckland District Health Board's acute mental health unit, Te Whetu Tawera.
Herald sources said he had been released from its care only days before the killing and the court heard today he was prescribed medication.
"In September last year he was unwell," Lummis said.
But, Lummis argues, Yad-Elohim's drug use led to the killing and he was well enough to be released from medical care.
"That [drug use] obviously hasn't assisted in helping those mental health issues," she said.
A urine test in September showed traces of meth and cannabis, she told the court.
The jury, of three women and nine men, will hear from several witnesses who were with Yad-Elohim on September 26, and experts, before deciding if he was insane at the time of the killing.
Lummis described the case as "crazy, odd, misguided" but said Yad-Elohim was not legally insane.
The trial continues.