Four days before his 23rd birthday, Corey Alan Breen wrote a letter to his mother, Debbie Breen.

"I get very angry and plot out murders of people I hate. I do it all in my head. I run over it again and again, refining every detail. They are always extremely horrific and extremely brutal," he wrote in August 2009.

"I'm scared someday soon the monster that I've kept locked away for so long will finally reach the surface. People will die and people will suffer."

Over the next 3½ years, Breen continued to make threats against his family, including his paternal grandfather Colin Breen, his sister Ashleigh Breen, his father Paul Breen and his stepmother Felicia Crawford.

He became fascinated with true crime stories and told his partner, Cassandra Fenech, he "had no soul and enjoyed seeing how people had died and the stories behind it".

Breen blamed his father for everything that had gone wrong in his life.

Paul Breen had an alcohol and gambling problem when Breen was a child and he separated from his wife Debbie in 1994.

However, he had since gone to Alcoholics Anonymous and had been sober for many years.

Breen sent his mother a text message in August 2012 that said: "Please tell Paul that if he tries to contact me again I will murder his whole f---ing family."

In February 2012, Ms Fenech took him to Wyong Hospital where he was assessed but found to have no acute mental health issues, psychosis or depression. Medical staff recorded his unresolved feelings of hate and anger towards his parents.

He later told Ms Fenech that he couldn't tell staff what he was really thinking.

A year later, he sent Ms Fenech a text message that said: "I can feel a massive f---ing skitz out coming soon and I don't know if I will be able to stop myself this time."

Six weeks later, Breen embarked on a terrifying crime spree that included carjacking, multiple assaults of his friends and bystanders, and the stabbing murder of his father and stepmother at their home in Killarney Vale on the Central Coast.

Breen, 28, pleaded guilty in June to murdering his 55-year-old father and 45-year-old stepmother on March 29, 2013.

A statement of facts tendered to the Supreme Court during a sentencing hearing on Friday revealed that, after his arrest, Breen told police he was on his way to kill his mother and her boyfriend.

He then planned to kill his brother Luke and sister Ashleigh.

He admitted he had driven to his grandparents' house earlier that evening with the intent of killing them but he was interrupted by a sensor light and thought it was his father arriving.

"I was going to butcher them … really don't know what happened with that … had my knife in my hand ready to roll.

"I thought sweet, cool, you know, kill a couple of birds with one stone."

As police urgently tried to track down Breen, he drove a stolen Holden Commodore to his father's house in Playford Road, Killarney Vale. He stabbed his father 15 times and his stepmother eight times before taking off.

Police were on their way to Playford Road when they saw Breen, covered in blood, driving near Wyong Road.

A pursuit followed, reaching speeds of up to 120km/h and narrowly avoiding several collisions.

Breed burnt out the clutch and the car broke down. He was chased on foot and surrendered.

A police officer told him: "You are under arrest for multiple stabbings and stealing a car. Do you understand that?"

Breen replied: "Multiple murder, mate.

"I achieved my mission."

At Wyong police station, he detailed the evening's events in a "calm, casual and factual way".

He said he had "absolute pure hatred" for his father because his father had beaten him when he was a child and his mother had not intervened.

Breen also said: "It's always sort of been like … it's a time bomb sitting on my head. I knew sooner or later I was going to go and f---ing get my old man and the rest of the c----s. Um, and just tonight was the night."

During a sentencing hearing on Friday, Breen's barrister Philip Young, SC, said his client had typed a two-page statement to be given to the court.

Forensic psychiatrist Olav Neilssen has assessed Breen and will be cross-examined by the Crown when the sentencing hearing continues next month.