Enraged by a decision to sell the family home, Patrick Francis Young beat his stepmother's brother to death with a wooden stick, a jury has been told.
Patrick Young, 43, is accused of murdering Aran Jenkanying, who died on January 30 2011, four days after being attacked outside the Otara home.
Ana Edmonds, 29, is accused of helping Young avoid arrest.
Young admitted intentionally causing grievous bodily harm to his stepmother, Arunee Jenkanying, when the trial at the High Court in Auckland began on Monday.
He denies murdering Mr Jenkanying, saying he acted in self-defence.
Crown prosecutor Susan Gray said Ms Jenkanying became the second wife of Young's father in 1998, but she lived in Australia for some time and never had a strong relationship with Young.
She returned in 2010 after hearing her husband became seriously ill. He died of cancer in September 2010.
Though Young and his siblings were attached to their family home, their father left the house to Ms Jenkanying and she made plans to sell it.
This incensed Young, who sent her texts calling her a dog and telling her to go back to Thailand, Ms Gray said.
When Ms Jenkanying and her brother visited the house on January 26 last year, Young hit Mr Jenkanying across the head numerous times with a stick, she said.
"Eventually he collapsed, and the accused continued to hit him while he lay on the ground."
He then attacked Ms Jenkanying in a similar manner, Ms Gray said.
Mr Jenkanying died four days later from brain injuries. Ms Jenkanying survived but suffered skull fractures.
Young went on the run for a few days with the help of Ms Edmonds before they were caught, Ms Gray said.
Defence lawyer Shane Cassidy said Young was set upon by the Jenkanyings.
"He reacted instinctively by defending himself from their aggression."
The trial is set to last two to three weeks.