Mrs Guy recalled the moment the damage was discovered. "I heard Scott say, 'Oh my God', then I saw the laundry door had been smashed. Then I saw the words."
She said the words were "so untrue".
"Deep down, I knew I hadn't done anything, which was why it was so scary."
Ewen Macdonald has admitted carrying out the vandalism and graffiti attack with friend Callum Boe. The Crown alleges he wanted to drive Kylee and Scott Guy off the farm he feared he was losing control of - and when that didn't work, he shot Mr Guy dead.
Macdonald has pleaded not guilty.
Mrs Guy said the damage was so bad she had to walk out of the house. It had been nearly finished and she and her husband had excitedly talked of nothing else for weeks.
"It was our whole life at the time."
The extent of the damage appalled her, as did how "violent" it was.
Almost every wall had been attacked with a splitting axe, windows smashed and plumbing fixtures ripped out.
"It was just terrible. We were asking, 'Who's done this? Who could do this?"'
The vandalism came months after Macdonald and Boe torched an old home on the Guys' farm that burned to the ground.
Mrs Guy said she and her husband talked about what happened with many people, including Anna and Ewen Macdonald - but she couldn't bring herself yesterday to say his name, referring to him only as "the accused".
She doubted she could move into the home. "When it first happened I was so scared ... I remember at the start saying I didn't know how I could live there."
Anna Macdonald, who is Scott Guy's sister, visited her husband in jail repeatedly, desperate to understand "what went wrong". That included asking him if he killed her older brother.
"He said, 'No, I swear I didn't'."
But he did tell her what was going through his mind when he carried out the vandalism.
"He said it was more directed at Kylee than Scott. He thought Kylee wound Scott up with his ideas and through ringing him up [while at work]."
Mrs Macdonald said her husband denied hating them, but said he was angry.
The trial has heard Ewen Macdonald didn't believe the farm partnership involving him and Mr Guy was fair. Anna Macdonald said he should have confided in her.
"I said to him, 'I can't believe that you felt so upset and cross about things and not being able to tell me about it' - because if I felt cross or upset I would have said ... And I just thought between a husband and wife that's what you do and the other person is there for you."
Her husband had said his silence was to protect her. "He said he wanted to keep me out of it. It was my family and I would be really hurt and [he] didn't want to involve me."
After the vandalism, she saw a change in his behaviour, believing he felt bad for what he'd done. He spent more time with the kids, went out of his way to be nice to Kylee and complained less about her and Scott, which was becoming a source of friction in their own relationship.
"He said he was sick of going on about it ... He started worrying more about what he was doing rather than what Scott was doing."
Mrs Macdonald said that when the police showed her pictures of the graffiti, she didn't think it was Ewen's handwriting.
"It probably never entered my mind that Ewen would have written this, so never compared it. I wasn't looking at the person next to me."
