The grieving father who lost everything in the massacre at Margaret River believes his four children – Kaydn, Rylan, Ayre and Taye – would still be alive if orders from the family court had been followed and enforced.

In an exclusive interview with Channel 7's Sunday Night programme to air tonight, an emotional Aaron Cockman claims specific orders over living arrangements were ignored with terrible consequences.

On May 11 at 4am, Cockman's former father-in-law Peter Miles walked from the main farmhouse to the converted shed where his daughter, Katrina Miles, and four grandchildren were sleeping.

He used a rifle to shoot all five of them as they slept in their beds, before walking back to the farmhouse to kill his wife of more than 40 years, Cynda Miles.

But what caused this psychotic snap and how did a loving grandfather manage to commit this unspeakable act?

Cockman tells Sunday Night it was the combination of a number of circumstances.

First, Peter Miles was supporting his daughter Katrina in a long and costly court battle over child custody rights.

They did not want Cockman to have access to his children.

As Cockman tells Sunday Night, it was an expensive exercise, with a high degree of emotional strain.

"Myself and the kids and Kat, and even Peter and Cynda suffered so much through the court system.

"And for Peter to actually take on a farm with all my kids on it and be the father figure, it's a lot of pressure on someone that is not mentally capable and shouldn't be in that situation".

The custody battle was traumatic, and Peter and Cynda Miles were heavily involved.

They applied for a Violence Restraining Order against Cockman, claiming he was following them.

The order was dropped and dismissed because of a lack of evidence.

Cockman says they told him on several occasions he would "never see his kids again."

He claims Peter Miles was also in turmoil over several family dramas including the current predicament of his son Neil, critically ill and in need of a kidney transplant.

Years earlier, his son Shaun had shot and killed himself in his early 20s and Cockman believes Peter couldn't take the prospect of another loss.

It was driving him deeper into depression.

But how could Miles pull the trigger?

On this, Cockman has a disturbing insight, recalling how Miles described "switching off" while killing sheep.

"I was out at the high school farm many times with him, killing sheep, and it's like, Peter, he just shut [his] mind off, and I know that's what he has done, he has shut his mind off, no feeling.

"Just like killing a sheep."