Michael Waipouri has been found guilty of murdering and kidnapping Waiuku man Lance Murphy.
The 56-year-old's body was found in a ditch near Wellsford in December 2015, nearly three weeks after he was last seen at a BP service station in Warkworth.
His long-term friend Waipouri stood trial for Murphy's kidnapping and murder.
Co-accused Steve Gunbie has been found not guilty of kidnapping and not guilty of being an accessory after he allegedly helped to dispose of Mr Murphy's body.
Waipouri will be sentenced on April 12.
"We've waited a long time for today's verdict, and Lance - our brother - was a loved husband, son, brother, grandfather, and friend who should still be with us today," said Mr Murphy's sister, Shannon Murphy.
"We'd like to thank the jury for their careful deliberations. We understand how difficult it would have been for them."
Mr Murphy was killed after Waipouri repeatedly hit him in the head with a bat and a tree branch on a remote hilltop in Puhoi.
He was handcuffed and had a bag over his head at the time.
Waipouri previously told the court he believed Mr Murphy was a hitman who'd killed 10 people.
He claimed Mr Murphy murdered his own wife and mother-in-law, and Waipouri feared he was next after he "declared war on him".
He said he then killed him after seeing a demon with 10 heads come out of his chest - one for every person Murphy had killed.
Waipouri's lawyers claimed he acted in self-defence, but Justice Anne Hinton has told the jury that cam no longer be considered.
During his closing address on Tuesday Crown prosecutor Gareth Kayes told the jury they could be well and truly sure that Waipouri is guilty of murder.
The court heard he used a bat to attack Mr Murphy and when that wasn't sufficient he switched to a tree branch.
"Waipouri didn't stop until Murphy's skull caved in, and he heard him stop breathing."
Mr Kayes told the court even that evidence alone proves his intention was to kill Murphy.
"We know what type of force he used. Waipouri later told police he strikes hard, and strikes for keeps."
With self-defence no longer able to be considered, Waipouri's lawyer John Munro urged the jury to instead return a verdict of manslaughter.
"I'm not asking you to return verdicts of not guilty to murder, we don't have self-defence anymore. But what you now have the ability to do is return a verdict of manslaughter. It's a middle ground and one step lower than murder. If you have any reasonable doubt, you can do that."
Mr Munro told the court Waipouri not once tried to cover up the killing.
"You may find some solace in that sort of honesty."
Justice Hinton summed up the case for the jury on Wednesday and urged them to put aside any emotion or prejudice.