A psychiatrist says it's impossible to predict the future behaviour of a man who stabbed his father and stepmother to death and wanted to kill his mother and siblings.
Corey Breen, 28, has pleaded guilty to murdering his father Paul Breen and his stepmother Felicia Crawford at their NSW Central Coast home on Good Friday 2013.
Giving evidence at a sentencing hearing, Dr Olav Nielssen says it's impossible to know whether Breen will want to harm his family again.
According to facts tendered to the Supreme Court, Breen had written to his mother in 2009 saying he plotted the murders and was scared "the monster that I've kept locked away for so long will finally reach the surface".
Dr Nielssen said it was likely Breen had suppressed the violent thoughts he harboured.
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But whether he would in future succumb to these again was "just impossible to predict", he said.
Breen stabbed his father 15 times before turning on his stepmother, stabbing her several times.
He had stolen a car to drive to his father's home, but unsuccessfully tried to carjack a 63-year-old man first, stabbing him in the face with a large hunting knife.
When police caught up with Breen that night, he said "I achieved my mission" and later confessed in an interview he had been caught "in the middle of it all".
"If the clutch didn't burn out 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 and whoever else was up there," he said.
Breen will return to court in October.
