An intoxicated Sydney prisoner who said "I will kill them all" before strangling and bashing a fellow inmate to death will spend at least five extra years in jail.
Mervyn Keith Davidson walked into Alfredo Pengue's Silverwater prison cell in February 2018, closed the door and subjected the 54-year-old to a "sustained and prolonged" attack, NSW Supreme Court Justice David Davies said.
When Davidson left the cell 25 minutes later, Pengue was unconscious on the ground, purple from the neck up and bleeding from the mouth.
His right eye was significantly swollen and almost every rib had been broken.
Davidson had consumed five Xanax doses and smoked ice the day of the attack and earlier told other inmates he'd "kill them all".
He was later found not guilty of murder because there was doubt over whether the intoxicated man had been capable of forming an intent to kill Pengue.
But Justice Davies rejected the 46-year-old's claims he was unable to remember the attack or was not guilty of manslaughter due to mental illness.
"This was not a fleeting event such as a shooting or a stabbing where an assailant with mental illness problems might not realise until too late that what he had done was wrong," Justice Davies said on Friday.
The judge found there was "little or no planning involved" in the attack and "no history of animosity (or) prior problems" between Pengue and Davidson.
Justice Davies ordered Davidson serve 11 years for the killing, including a seven-year minimum term to begin in September 2022.
That later start is due to Davidson already serving time for choking a fellow prisoner with a T-shirt in February 2017.
That offence's minimum term expires in September 2024.
Justice Davies took into account the significant and chronic problems caused by Davidson's dysfunctional upbringing and his early and ongoing exposure to violence, heavy alcohol use and drugs.
A test of his verbal comprehension, perceptual reasoning and IQ placed him below 99 per cent of the population, the judge said.
Davidson's violence means he's spent every day in solitary confinement since the attack on Pengue.
Justice Davies found special circumstances arising from Davidson's institutionalisation and the "very considerable" assistance he would need to re-integrate into the community.
But he held reservations Davidson would stay away from drugs if ever released into the general population or on parole.
"In such circumstances, his risk of reoffending would be moderate to high," the judge said.