“I still love who Peter was. If it wasn’t for him, I wouldn’t have Katrina, I wouldn’t have her kids,” Aaron Cockman told reporters today.
“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 him off from the kids".
"I had so much anger in the last few years," he said.
"The anger towards them now, is completely gone, I don't feel angry, I feel tremendous sadness for my kids, but I don't want anyone to feel anger.
"I still love who Peter was. If it wasn't for him, I wouldn't have Katrina, I wouldn't have her kids.”
Cockman said Miles did not make a "snap" decision to kill his wife, daughter and grandchildren.
"Peter didn't snap. He's thought this through. I think he's been thinking this through for a long time," he said.
"All these kids died peacefully in their beds. The [police officer] that went through, he said they looked all peaceful. How the hell Peter did that I still can't figure out, but if someone did it ... he did a good job, he did a really good job."
The tragedy is being described as Australia’s worst mass shooting in more than two decades.