Tarapata never disputed that he killed the two men as they ate noodles in the lunchroom.
However, his lawyer, Jonathan Krebs, argued that his client was legally insane 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.
During the trial, the jury listened to Dr Jeremy Skipworth, the defence's expert witness and the clinical director at the Mason Clinic for forensic psychiatry.
He concluded that Tarapata was insane.
A second psychiatrist, Dr Justin Barry-Walsh, also agreed with Skipworth and said Tarapata was "floridly psychotic" at the time of the homicides.
Psychiatrist Dr David Street, who gave evidence for the Crown, was called to give rebuttal evidence and told the court that Tarapata's thoughts were fluid during the killings.
But Tarapata, a diagnosed schizophrenic, had a history of having religious delusions and was also known to make daily sacrifices to God by burning meat in his backyard.
He claimed God spoke to him and instructed him to kill Matthews and Fanning, believing they were having an affair with his partner Tamara Cassie.
Cassie, who also worked at the store, never had a sexual relationship with either man.
Krebs argued that because his client believed God was the highest moral authority he lacked any awareness for the wrongfulness of his actions.
The Crown said that Tarapata knew what he was doing and was overcome by jealousy.
Krebs told the court during his closing submissions that 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.
On the day of 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 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 testified.
The pair then returned to their car and left the scene.
Tarapata, seeking direction from 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.
Crown prosecutor Richard 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 in to the police station at midnight on the day of the attacks.
