The shattered father of the four children killed in the Margaret River family murder-suicide says their grandfather didn’t snap before it’s believed he shot them.

Peter Miles, his wife Cynda, daughter Katrina and Mr Cockman’s four children – daughter Taye, 13, and sons Rylan, 12, Arye, 10, and Kadyn Cockman, eight – were found dead at the Miles’ Osmington property, northeast of Margaret River in Western Australia on Friday.

Peter Miles had not been named by police as the killer on Sunday afternoon, but the grieving father left no one in doubt that he regards him as responsible for the horrific crime.

“Peter didn’t snap – he’s thought this through. I think he’s been thinking this through for a long time,” Aaron Cockman, speaking for the first time since the tragedy, told reporters.

“All the kids died peacefully in their beds.

“They looked all peaceful. How the hell Peter did that, I still can’t figure out. He did a good job.

“He did a really good job.”

Mr Cockman said he still loved Miles for “who Peter was”.

“If it wasn’t for him, I wouldn’t have Katrina, I wouldn’t have her kids,” he said.

“So it’s not some random guy off the street who’s taken them away from me – he gave them to me and now he’s taken them away.

“If it had to happen, there is no better person than that.”

Mr Cockman, who was estranged from Katrina, said he had not spoken to Peter and Cynda “since they cut me off from my kids” but being angry served no purpose.

“Anger will destroy you. I’m sad but I’ll get through this.”

Mr Cockman said he would never forget his children’s personalities.

“I could talk about my kids all day — Taye is a beautiful girl,” he said.

“I’ve lost everything in my life.”

-AAP