Killing David was an act of self-defence, lawyers have claimed during the opening arguments of Haig's murder trial.

David, 31, was shot and killed at the Geelong family home he shared with Haig and their parents in October 2020.

Haig Arslanian, then 39, admits he is to blame for his brother's death. He admits he fired the gun that ended David's life.

But he denies he is guilty of murder, claiming the October 5 shooting was necessary to prevent himself being killed or seriously injured.

The brothers were no strangers to arguments and fights.

There was growing frustration, anger and resentment between them, prosecutor John Dickie said.

"The accused perceived that David was using hard drugs and he was gambling," he said.

"He understood that David had stolen money from him and other family members (and) that David was not showing family members the respect he should."

Just weeks earlier Haig thought he was going to die when David tried to strangle him, defence barrister Jarrod Williams told the Victorian Supreme Court on Wednesday.

Haig told police he was scared of being hurt by his brother, who used his size against him. David, at more than 190cm tall and about 170kg was substantially bigger than Haig, Mr Williams said.

"He said he felt intimidated, threatened and scared of his brother," he told jurors, sitting in Geelong.

"That fear didn't just arise out of nowhere ... there was a history of David being violent toward Haig."

Their parents, who also have two other sons, are expected to give evidence about the September 2020 fight during the trial.

On the Monday evening Haig, after having his head shoved through a mirror by David, went and got a shotgun he had earlier stolen from his father's gun safe, jurors were told.

Haig later told officers he was shaking, sweating and throwing up as he returned with the gun. He said he didn't intend to shoot David, but to scare him off.

"He told police that even though he had the gun David continued to come at him, continued to taunt him," Mr Williams said.

He was just metres away when Haig fired.

"Tragically, sadly, Haig shot his brother David and there's no question about that," he said.

"The focus of this trial ... is why Haig did what he did."

The trial before Justice Rita Incerti is continuing.

Australian Associated Press