A Perth man who fatally stabbed his son laughed and joked with police as he told them he thought he did "the right thing" because the 23-year-old was a "complete arsehole" who regularly physically assaulted him and "terrorised" his family.

Ernest Fisher, 67, made the comments in an interview with police recorded four weeks after Matthew Fisher-Turner was last seen alive on AFL grand final day in October 2016.

The interview was shown to the WA Supreme Court, where Mr Fisher is on trial for his son's murder and two of his other children, Joshua, 28, and Hannah, 21, are facing charges for allegedly trying to cover up the crime.

Mr Fisher initially said his son had told him he had travelled to the eastern states with a "hot chick", but after about an hour-and-a-half of questioning, he finally admitted "it was me, I'm the one who killed him. I'd had enough of him".

He asked police "if they had found him", and when they replied "no", he told them "he is in the back garden".

Stabbing own son 'not pleasant'

At times in the interview Mr Fisher laughed as he recounted going to the shed to get two knives which he used to stab his son as he stood in Joshua's bedroom.

"I knew I had to get the jump on him, he was powerful and agile," he said.

"I wanted to get the right angle. He's strong and he's fast. I think I stabbed him in the back first."

Ernest Fisher says he killed his son because of his threatening behaviour. ( Supplied: Anne Barnetson )

When asked how hard he had stabbed his son he replied: "Hard, because I wanted it to go through. I had to make it count, because if he came at me I would be dead."

"It wasn't a very pleasant sensation, stabbing your own son," he said.

"I think I've done the right thing in a twisted sort of way. He was an arsehole."

Police asked him how many times he had stabbed his son, and he replied: "You need to dig him up and find out."

When told he was now going to be under arrest for murder, he laughed and quipped: "I can't go back."

'Footy was a priority'

Mr Fisher said the stabbing happened after Mr Fisher-Turner had taken his car keys at a time when he wanted to drive to his friend's house to watch the grand final.

"Footy was a priority. I didn't want want to miss it," he said.

He said he started to dig a hole in the backyard before leaving the house and going to his friend's, where he stayed for about five hours.

When he got home, "Matt was in the ground."

Joshua and Hannah Fisher-Turner are accused of burying the body and cleaning up the crime scene.

Matthew Fisher-Turner's body was buried in the family's backyard. ( Facebook: Matthew Fisher-Turner )

Mr Fisher said he believed the other members of his family were "relieved" afterwards, claiming Matthew was using "a cocktail of drugs" and describing him as "a horrible lad" and "like a terrorist".

"You can't imagine what it's like being threatened on a continuing basis and being hit. That's the main reason," he told police.

Mother's death a catalyst

Mr Fisher said his son had always been a "black sheep" but after the "traumatic" death of his wife in March 2014, his son's behaviour became worse.

"Once Jill died, it escalated because with her around he was on good behaviour," he said

"He blamed me for not making her go to hospital, but it was her decision."

Mr Fisher rejected suggestions he had killed his son out of spite or revenge, saying he "couldn't think of anything else" to resolve the situation.

"It was sort of the last option," he said.

"It's not very nice to kill your son to protect your family and yourself. I was upset when I first did it, Christ yeah."

The detectives also suggested that Mr Fisher could have temporarily moved his family out of the house to get away from Matthew, but he replied: "I didn't want to waste money."

"I've got my own house, I own it … I didn't like the idea of moving," he said.

Mr Fisher has pleaded not guilty to the murder charge and claims the stabbing of his son was done in defence of himself and his other family members.

The trial is expected to finish later this month.