Anna Macdonald confronted her husband after he admitted burning down her brother Scott Guy's house and asked, "How could you hate someone to do this?''
The Crown alleges Ewen Macdonald, 32, carried out the arson attack in October 2008 as part of a campaign of intimidation aimed at frightening Mr Guy and his wife Kylee away from the family farming business.
Macdonald has admitted the arson but denies shooting and killing his brother-in-law Mr Guy, 31, in the driveway of his Feilding home on July 8, 2010.
Giving evidence today for the third time during her husband's trial in the High Court at Wellington, Mrs Macdonald said she asked her husband why he committed the arson.
"Once he was arrested I had a lot of questions on why things happened and how he felt about things. I didn't know so I said to him, `Why did you do this, how could you hate someone to do this?'''
Macdonald said he did not hate them and he did it because it was fun and to "annoy them''.
Before Macdonald's arrest, there was nothing to suggest he had been involved.
Mrs Macdonald told the jury she would have expected "a change'' in her husband but there was none.
The old house on the Aorangi Rd property had been sitting on a trailer and was due to be shifted so a new house could be built.
On the night Macdonald and a young accomplice burnt it down, Mrs Macdonald could not recall any noises or sirens.
She also could not recall what Macdonald had done that night or whether he had left the house.
In a written statement read to the court, Kylee Guy said she and Scott had originally wanted to do up the old house but it involved too much work.
They sold it to a removal company for $8000 and received a $2000 deposit, with the rest to be paid once the house was shifted off the property.
Mrs Guy said she was "devastated'' when Scott told her the old house was destroyed.
The Guys later built a new house on the property.
Mr Guy's father Bryan Guy read out a statement in which he said the arson had disrupted the construction of the new house.
"I don't think Scott and Kylee had any emotional attachment to the old house, so I don't think they saw it as any huge loss. That's my impression.''
The court earlier heard evidence about three chocolate labrador puppies that were discovered missing from a cow shed on the Guys' property after Mr Guy was killed.
The Crown alleges Macdonald stole, killed and buried the puppies to throw investigators off the trail.
Also giving evidence for the third time today, Kylee Guy said her husband had given her a "beautiful'' labrador called Katie just before they were engaged.
Katie had a litter of eight puppies which were seven to eight-weeks-old when her husband died.
One of them was given to a close friend, Joanne Moss, and the rest were to be sold for $700 each.
Mrs Guy fought back tears as she read aloud a flier they had made offering the puppies for sale.
She said she was "shocked'' when she found out the puppies were missing.
The court heard evidence that Macdonald was the first to alert police that the puppies were missing.
Mr Guy's older sister Nikki Guy said he returned from feeding the puppies the night after Mr Guy was killed in an "animated state''.
Macdonald said three puppies were missing.
"He said we weren't allowed to say anything to Kylee.''
Ms Guy said he told them it would upset her too much.
The family discussed theories about how the missing puppies could be linked to Mr Guy's death.
Bryan Guy said police asked the family not to mention the missing puppies because they did not want information that could help the investigation made public.
"The fewer people that knew about the puppies the better, was my understanding.''
He visited the puppies the day before Mr Guy was killed, when his son showed him the work he had done at the cow shed.
"The last thing I said to Scott was, `I'll see you in the morning.'''
The trial before Justice Simon France and a jury of 11 continues tomorrow with evidence on intimidating letters sent to the Guys before Scott's death. Macdonald has denied responsibility for the letters.By Matthew Backhouse @Mbackhouse Email Matthew