Anthony Robert Harvey wrote about embracing his "darkness and animal instincts" before murdering his three young children, wife and mother-in-law with newly purchased knives.
The killings were so degrading, callous and abhorrent that West Australian Director of Public Prosecutions Amanda Forrester argued the 25-year-old should never have any hope of being freed from prison.
"This is a rare circumstance where the nature of the offending is such that that should be removed," she told the WA Supreme Court on Friday.
Two-year-old twins Alice and Beatrix, three-year-old Charlotte and their mother Mara Lee Harvey, 41, were killed at their Bedford home on September 3, 2018.
Beverley Ann Quinn, 73, was murdered when she visited the next morning.
As the details were read in court, Harvey put his head in his hands and covered his ears.
Crying was also heard from the public gallery.
Ms Quinn and Ms Harvey were struck on the head with a pipe and repeatedly stabbed with a large knife - almost the size of a machete - Harvey had bought days earlier.
Harvey had consumed wine and said he quickly tried to "finish" his wife of three years, stabbing her at least 12 times.
The children were murdered with a smaller knife while they slept because "they were just little girls".
Charlotte was stabbed 38 times.
Ms Harvey and her children were placed as though they were cuddling, then covered with a doona with flowers on top and toys surrounding them.
Ms Quinn was covered with a doona in the kitchen.
He also took photographs of them.
"To my beautiful wife, I'm so sorry. I would give anything to undo what I've done," he wrote in a note.
"I think I've lost my mind.
"Take care of those little girls like you always do. I love you so much."
Harvey slept after the killings and said when he woke up it felt like a nightmare, except nothing had changed.
He remained at the house for days and lied to his wife's employer to explain her absence.
Harvey also sold items and took money from his wife's bank account before travelling about 1500km north to the Pilbara town of Pannawonica where he saw his parents.
"I've done something really wrong," Harvey said.
"I hurt them ... I miss them ... It was Father's Day the day before."
With the help of his father, Harvey turned himself in to police on September 9.
He told authorities he had not been mad or angry, describing his marriage as good and his wife as fantastic.
Ms Quinn was also described as caring and he said he got along well with her.
Ms Forrester said it was a premeditated attack and gross breach of trust, noting Harvey purchased the weapons on August 23 and 29, and had written a journal entry about his plan to "eliminate" his family.
"I am no psycho. I feel. I feel too much, I always have ... I will regret what I do," he wrote.
Ms Forrester also noted the catastrophic effect of "wiping out" a family on the remaining extended family.
"It is inexplicable violence against people who were entitled to the offender's protection (and) his love," she said.
Defence counsel Sam Vandongen said his client was young and remorseful, urging Justice Stephen Hall not to order he never be released.
The court also heard details about Harvey's mental health and a suggestion he had symptoms consistent with high-functioning autism.
Justice Hall said he wanted to limit the family's continued suffering but had matters to consider before sentencing.
Harvey will he sentenced on July 19.
He could be the first person in WA ordered to never be released under laws changed in 2008.