Notorious murderer John Walsh repeatedly ploughed a sandwich press into his cellmate's head because he thought he was "an arsehole" and had the look of a man who was planning something.
"He just died, it was so easy," the 79-year-old told police after he murdered fellow convicted killer, Frank Townsend, in their Long Bay prison cell in early January 2017.
"I stepped forward with both hands and choom, onto his face.
"I don't work in anger, I work in tactical ... cold rage."
Walsh and Townsend had shared the cell for five days in the jail's aged care unit for elderly and frail offenders, according to the agreed facts, signed by Walsh before his NSW Supreme Court sentence hearing on Friday.
Prison guards were on patrol when they heard about six "bangs" or "thuds" - each a second apart.
One shone a torch into the pair's cell and asked: "What's going on?"
Walsh was silent and sitting up while Townsend was making a "loud snoring noise" from his bed.
The officers left, locking the door behind them, but heard two similar thuds 30 seconds later.
"I think I hurt him," Walsh said, having struck Townsend at least eight times.
A metal sandwich press was found on the floor wrapped in a blood-soaked pillow.
Doctors later noted Townsend's skull was so severely injured his cheek seemed "concave" and partly missing.
Walsh, who is serving two life sentences plus 12- and 15-year jail terms, pleaded guilty to the murder in June.
Crown prosecutor Sharon Harris on Friday argued he should receive life imprisonment for the "ferocious" killing on a defenceless victim.
"Your honour, some people are just bad," Ms Harris said to Justice Lucy McCallum.
Townsend's two daughters were in court on Friday, as their father's bearded killer sat silent in his prison greens.
In June 2008, Walsh stabbed his wife and bludgeoned her and his seven-year-old grandson with a hammer.
He drowned his five-year-old granddaughter in a bath at the Cowra home and also drowned the family dog.
Walsh's daughter came to collect the children and fought off his axe attack but suffered serious head injuries.
When asked what he was thinking when he hit Townsend, Walsh replied: "I don't think I was thinking anything".
"All I remember is bang, bang, bang ... I shut it out probably," he told police.
"The only thing I can't shut out is, my own family ... I don't know why that's happened because there was no anger, no drugs, no booze, just that depressing silence and I went and killed my wife.
"Maybe some people shouldn't be born."
The facts state he has no contact with his three siblings or two children and doesn't know if they are still alive.
Defence barrister Janet Manuell SC said Walsh suffers from a mild cognitive disorder and has hearing aids but his mental health "presentation" hasn't changed in a decade.
Walsh will be sentenced on August 23 before Justice McCallum who convicted him over his three other killings.