A man killed in broad daylight in an Adelaide park was "in the wrong place at the wrong time" with his alleged murderer "ready to snap", a court has been told.
Shaun Russell, 36, was walking home through Adelaide's Hackham West Reserve after watching some of the AFL Grand Final at a friend's house in September, 2019, when he was beaten with such a degree of force that it fractured his skull.
Robert Joseph Patard, 45, has gone on trial in the South Australian Supreme Court for Mr Russell's murder.
Prosecutor Jemma Litster told the jury that Mr Patard was in a "volatile and angry state" after arguing with his partner for most of the day in the Hackham West Reserve.
Ms Litster told the court that Mr Patard "simply erupted" when Mr Russell walked towards the couple, pulling up a large wooden garden stake and beating him with it at least three times.
"Mr Russell was not known to the accused. He was simply in the wrong place at the wrong time," she told the court.
"[Mr Patard] quite deliberately, and aggressively, went on the offensive against Mr Russell.
"It was a one-sided attack.
"It was simply a large and aggressive act of violence."
Ms Litster said Mr Patard and his partner had been arguing aggressively in the park for most of the day.
"The accused had become fixated on a paranoid belief about [his partner's] sexual interactions with other people," Ms Litster said.
"He had convinced himself she had been raped. He became angry at her for that.
"She repeatedly denied being raped and he ranted about it and so it went on that day."
Ms Litster said police officers and council workers had both checked on the wellbeing of the couple throughout the day.
She said that, at one point, Mr Patard insisted on inspecting his partner's body for signs she had been raped in the park in "broad daylight".
It was shortly after, she said, that Mr Russell walked past.
"Mr Patard was in a [fractured] state of mind. He was ready to snap in anger," Ms Litster said.
"After the assault on Mr Russell, Mr Patard then moved quickly to remove himself from the scene.
"He left Mr Russell, badly injured, bleeding [from] the head and lying on the ground.
"He disposed of the stake in a concealed area near some trees in the reserve."
The court was told Mr Patard then told a couple and child entering the reserve not to go in there and that "something's going on in there, you don't want the little man to see".
"He did all those things to avoid detection for what he had done," Ms Litster said.
Council workers later found Mr Russell's bloodied body in the park and contacted emergency services.
Ms Litster told the court that, hours after the alleged murder, the couple drove to Kadina and slept in their car before returning to collect their caravan, which they slept in until Mr Patard's arrest on October 1, 2019.
The court was told that, when police arrested him, Mr Patard had a burn injury from burning clothing.
After Mr Patard's arrest, the court was told, he "discussed his involvement with the crime" with several family members and friends.
Ms Litster told the court that Mr Patard initially blamed his partner for Mr Russell's death, before "his explanation shifted to self-defence".
"He said he bashed the deceased, he didn't intentionally kill him and when he left him he was still alive," Ms Litster said.
"He said, 'Well, it was self-defence, really. You know what, I'm not going to let someone beat me up. You wouldn't either'."
In a later call, she said, Mr Patard said he would say he was on drugs at the time.
Mr Patard's lawyer, Nick Healy, told the court there was no dispute his client had killed Mr Russell but the question for them to consider was whether it was lawful and whether Mr Patard was acting in self-defence.
The trial before a jury of 15 is expected to run for three weeks.
