A jury will decide if the Bible and God commanded a "disciple" to stab two shop workers to death or if it was a delusional and jealous attack.

Paul Matthews, 47, and Paul Fanning, 69, were stabbed to death at the Takanini Ezy Cash store, where they worked, on July 19, 2014.

Fanning was stabbed six times to his chest and neck, and Matthews was stabbed 15 times. His throat was cut.

Zarn Tarapata, 27, was their killer and has been on trial in the High Court at Auckland during the past two weeks charged with their murders.

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Today closing submissions in the case are being heard.

Tarapata accepts he killed the two men as they ate noodles in the lunchroom, but his counsel, Jonathan Krebs, argues Tarapata was legally insane at the time and believed he was carrying out "God's work".

Under New Zealand law a defendant accused of a crime is presumed sane and the burden of proof lies with the defence to prove insanity.

Tarapata, a diagnosed schizophrenic and known to make daily sacrifices to God by burning meat in his backyard, claims God spoke to him and instructed him to kill Matthews and Fanning for their sins - an affair with his partner Tamara Cassie.

Krebs argues that because his client believed God was the highest moral authority he lacked any awareness for the wrongfulness of his actions.

Cassie, who also worked at the store, never had a sexual relationship with either man.

The Crown argues Tarapata knew what he was doing and was overcome by jealousy.

"You may have some views about whether he should be in a mental institution or prison," Crown prosecutor Richard Marchant said during his closing address today.

"You may feel some prejudice towards the defendant because he accepts he killed two men."

But Marchant called on the jury to assess the evidence "clinically and dispassionately".

"You will conclude that the defendant entered the building armed with at least two knives and stabbed [these men] to death because he was angry and jealous," he said.

"The defence largely relies on what the defendant said eight months afterwards and longer - years later."

Marchant added that "more often than not" a person with schizophrenia will appreciate what they were doing was morally wrong.

Dr Jeremy Skipworth, the defence's expert witness and the clinical director at the Mason Clinic for forensic psychiatry, said in his opinion Tarapata was insane.

A second psychiatrist, Dr Justin Barry-Walsh, agreed with Skipworth and said Tarapata was "floridly psychotic" at the time of the homicides.

But psychiatrist Dr David Street, giving evidence for the Crown, believes Tarapata's thoughts were fluid during the killings.

Krebs told the jury during his closing address that it was "possible to be angry and or jealous as well as being told by God to do something".

"It's not as simple as accepting one or the other, it's probably a case of both," he added.

Krebs said the case was a "tragic situation of a man who was so unwell he was hallucinating commands from God".

He also said it was possible for his client to have understood the illegality of his actions but not the morality.

Moments before the killings Tarapata had arrived at the Great South Rd shop with Cassie and their children.

Cassie entered the store to have her cellphone repaired and told Tarapata to stay in the car with her children, the court has heard.

But Tarapata left the car and snuck around to the rear of the business.

He entered the lunchroom and stabbed Matthews and Fanning to death over about three minutes.

A "wild and possessed" Tarapata covered in blood and clutching a knife was then seen coming down the hallway, Cassie has testified.

The pair then returned to their car and left the scene.

Tarapata, reading his Bible, then told Cassie to drive around West Auckland.

They stopped near the Avondale Racecourse and Tarapata washed his hands, clothes, and seemingly also made efforts to discard the knife.

Marchant questioned why the "disciple of God" would make attempts to hide evidence had he believed his actions were justified.

But Krebs rebutted the Crown's claim that Tarapata was attempting to run from police, and rather "wanted time in the car to think and consult his Bible".

Tarapata was arrested in Huntly after walking into the police station at midnight on the day of the attacks.