Man killed granny believing she was his abusive stepdad

After suffering years of torturous abuse at the hands of his stepfather, Hayden Kidd's grandparents' home was an oasis of calm.

But it was inside that safe haven where he violently killed one of the people he loved the most.

Believing his grandmother Shirley was his former stepdad, Kidd slaughtered her using a garden pick.

He did so after suffering years of "cruel and sadistic" abuse at the hands of the man he thought was his father, his lawyer told the Supreme Court in Melbourne on Tuesday.

His three siblings had received different treatment and everything unravelled for Kidd when he found out the truth.

"Mr Kidd was tortured by his stepfather as a child," defence barrister Tim Marsh told a pre-sentence hearing.

"As a young child he was left utterly bewildered but with no explanation as to why he was being singled out for that treatment.

"The moment the penny drops and he's told ... the person who did those things to you was not in fact your father, it's rare to come across abuse that is quite so cruelly targeted."

Kidd, 22, pleaded guilty to manslaughter in April, days after his murder trial began at the Supreme Court in Ballarat.

In a "bizarre confluence" of drug-induced psychosis and PTSD, he walked into Shirley's bedroom and found her asleep with his seven-year-old cousin, Mr Marsh said on Tuesday.

"Psychosis gives his thoughts delusional form, PTSD is the rocket fuel that gives the energy, and these two combined to such an extraordinary affect," he said.

Believing the boy was him and Shirley was his abusive stepfather, Kidd grabbed his cousin and ran to the garage.

In a delusional state after smoking cannabis, Kidd was trying to fend himself from Shirley when he attacked her with a garden mattock.

The 67-year-old woman was found dead inside the garage, lying in a pool of blood.

Kidd moved in with his grandparents when his mother left the stepfather, and returned to live there when he was 18, Mr Marsh said.

Their home was "an oasis of calm away from the chaos" Kidd suffered throughout his childhood, he said.

"These actions have destroyed the only sanctuary Hayden has ever known," Mr Marsh said.

He said Kidd's biological father had died from cancer shortly before the killing.

Mr Marsh acknowledged Shirley's death was a "an unusual and tragic case" but said the seriousness of his offending fell away when Hayden's explanation was taken into account.

He asked for Kidd to be assessed for a community work order combined with a prison sentence.

Prosecutor David Glynn urged the judge to remember a grandmother's life had been taken and a seven-year-old boy had witnessed it, calling for a longer prison term.

"What happened here is that a grandmother unlawfully and wrongfully lost her life," he said.

"You couldn't imagine a more grave example of manslaughter."

Kidd closed his eyes as details of the killing were read to court and as he heard statements about the impact of the crime on his family.

His uncle Matthew cried as he detailed his heartbreak over losing his mother, his "biggest influence and best friend".

"I've become an empty shell of a man that seems to be fighting to survive daily rather than living," he said.

Justice Michael Croucher will sentence Kidd on August 1.

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