From the moment his son smashed through the front door of his NSW Central Coast home with a hunting knife, Paul Breen began screaming.
Within minutes he had been stabbed 15 times with "severe force" before his son Corey Alan Breen turned on Paul's wife - Breen's stepmother.
Felicia Crawford, having seen the murderous rage Breen was in, ran and hid.
But Breen found her, stabbing her furiously in the back, the head and the neck.
"Their last moments must have been filled with unimaginable fear and horror," Acting Justice Jane Mathews said in sentencing 29-year-old Breen on Wednesday.
Breen pleaded guilty to murdering his father and stepmother at their Killarney Vale home, north of Gosford, on Good Friday in 2013.
As he was sentenced to at least 33 years and a maximum of 42 years, his family members shrieked abuse at him in the Supreme Court.
Outside court, Paul's brother Tony said he was worried his nephew would kill again on his release.
Breen's mother Debbie said "I don't know how to feel".
Breen, the court heard, had blamed his father for many of the things that had gone wrong in his life.
Paul, who was 54 when he was killed, had a history of gambling and drinking problems, and had been violent towards his family, including Breen.
But in the decade before his death he had been sober and reconciled with his other children.
Breen, however, still harboured deep resentments, and on the night of the murders the "time bomb" ticking inside his head went off and the "monster finally reached the surface", Justice Mathews remarked.
Breen drove to his grandparents' home where he planned to kill them but when that attack didn't take place he went to his father's.
Using the butt of a hunting knife, he smashed the unlocked front door's glass panels and marched inside where Paul was screaming.
After he killed Paul and Felicia he was spotted by neighbours calmly getting back into a car.
When police caught up with him he immediately confessed - saying "I have achieved my mission".
"I was on my way up to my mother's house ... I probably would have knifed her and her f***ing boyfriend and my brother and sister," Breen said.
While Justice Mathews said it was "extremely difficult if not impossible to envisage any rational motive for these extraordinary events" - a "highly chilling" letter sent by Breen to his mother in 2009 provided some insight.
Breen wrote how he plotted the murders of those he hated and was "scared someday soon the monster I have kept locked away for so long will get out".
Justice Mathews said something inside Breen snapped that Good Friday afternoon.
"It is impossible to know precisely what it was but it was probably connected to the alcohol and drugs that he consumed."
She said she would impose the maximum sentence of life imprisonment, finding that he had - since the murders - shown deep remorse for his actions, had pleaded guilty and had reasonable prospects of rehabilitation.
He will be eligible for parole in 2046.
