Marcus Bax slit the throat of a Northland father who he looked up to as a role model and a father for many years.

The 39-year-old lived with Peter Nilsson's family and was treated as part of them for many years until August 4, 2016, when "impulse" took over and murdered him in Te Kao.

Bax pleaded guilty to murder and was yesterday sentenced in the High Court at Whangarei to life in prison with a minimum term of imprisonment of 15 years.

Mr Nilsson's two sisters and a brother, along with his daughter Anne Marie, attended the sentencing and read their victim impact statements in court.

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Bax threw a large chipper bar at Mr Nilsson, 77, before killing him with a butcher's knife after driving to his house following dinner.

He returned to the house he shared with Ms Nilsson and her family, including her brother, a few kilometres from where their father lived, covered in blood holding the knife.

Bax told them what he did and asked for help in destroying evidence.

In her victim impact statement, Ms Nilsson said she had known Bax for more than 20 years and trusted him to play with her daughter and to take her swimming, hunting and to the beach.

She said Bax told her his actions in killing her father made him feel powerful and that he enjoyed that feeling.

"I want to hate you but you're just an alien to me."

Ms Nilsson said words could not express the pain and anguish of her family from Bax's actions without any regard to others.

Her family struggled to manage the farm as there was no handover, she said.

Crown solicitor Mike Smith said there was a high level of callousness and brutality against a 77-year-old man in his own home.

It was a deliberate and conscious attack on a vulnerable man in an isolated area where police help could be an hour away.

In an apology letter Bax wrote to Mr Nilsson's family, he hoped one day he could apologise to them in person and asked for their forgiveness.

Justice Edwin Wylie said Bax considered Mr Nilsson as a role model and a father but impulse took when he murdered him.

The court did not take into account Bax's previous convictions that were mostly for driving and disorderly type offences.

Although Bax still suffered from a number of mental health issues, he had been declared fit to stand trial.