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When he committed his fourth murder, John Walsh was a 77-year-old prisoner serving life in NSW for slaying his wife and two grandchildren.
After using a sandwich-press to bludgeon his cellmate to death, Walsh told police: "I wasn't in a temper, I never have been" and "I'm not bloody sorry because he's an arsehole".
In imposing another life sentence on the now 79-year-old, Justice Lucy McCallum said the circumstances of the cold-blooded, calculated killing "bear an eerie similarity" to his three previous murders.
"He describes acts of murder as if he were explaining how to change a tyre," she said on Thursday.
Walsh pleaded guilty in the NSW Supreme Court to murdering Frank Townsend in their Long Bay prison cell in the aged care unit for elderly and frail offenders in early January 2017.
"It involved repeated acts of extreme violence and ferocity causing catastrophic injury," the judge said.
"The victim was completely vulnerable and must have endured great pain."
In 2009, the same judge jailed him for life - meaning he will die in jail - for slaying three relatives the year before in their Cowra home.
"It does not follow that the imposition of a further sentence for this fourth murder committed by him is an arid exercise," she said.
She noted the importance of punishing him separately for taking the life of Mr Townsend, acknowledging the suffering his relatives had endured.
Walsh had "methodically" stabbed his wife and bludgeoned her and his seven-year-old grandson with a hammer before drowning his five-year-old granddaughter and the family dog in a bath.
When his police-officer daughter returned home he pretended nothing was wrong and made her a cup of tea before attacking her from behind with an axe as she discovered her children's bodies.
His "capacity to pretend nothing was wrong" was echoed in his latest murder.
When prison officers came to his cell after hearing noises, they found Walsh silently sitting on his bed while his cellmate made an apparent loud snoring noise but when they left he resumed his attack.
A metal sandwich press was later found on the floor wrapped in a blood-soaked pillow case.
"(His) glib description of his conduct on each occasion shows not the least remorse and indeed indicates that he is incapable of remorse," the judge said.
"The conclusion that, even as an old man, the offender poses an intractable threat of unprovoked, deadly violence is inescapable."