THE brother of an alleged double murderer has told a Hobart court of the heartbreaking circumstances that led to the killing of his mother and stepfather.
West Australian man Nicolau Francisco Soares, 28, has pleaded not guilty to killing Dr Delys Weston and Professor Gavin Mooney at their Mountain River property in the state's south in December 2012.
The trial before Justice David Porter began yesterday in the Supreme Court of Tasmania, and defence lawyer Rochelle Mainwaring said the case was "sad and tragic".
"It is strikingly clear Mr Soares was so mentally unwell his behaviour fits into the insanity provisions of the law," she told the court in opening.
Crown prosecutor Linda Mason told the court the two academics were attacked by their son, who was armed with a hammer, while they were eating dinner.
"You'll be left with no doubt that this is a horrific crime," Ms Mason told the court.
The court heard Dr Weston had convinced the accused to move to Tasmania and live with her and her husband at Mountain River, but soon after the move his mental health "took a turn for the worse".
Alex Soares, the younger brother of the accused, described to the court his brother's battle with a diagnosis of mental illness, and his "sporadic bursts of delusions" that started about four years ago.
He told the court his brother had started getting abusive towards his parents before their death.
"Mum said Nic was at crisis point," he said.
Mr Soares said his brother's symptoms were "played down" by health professionals, and on the day before the alleged crime, Professor Mooney had been trying to get him schizophrenia medication.
Mr Soares described growing up with his brother, who he said was a shy but intelligent child, a champion gymnast and later an accomplished guitarist.
"Behind closed doors his behaviour became more erratic," Mr Soares said.
Mr Soares said his brother experienced paranoia and anxiety, and regularly feared for his life but had no insight into his mental health issues.
He told the court his relationship with his brother was "extremely close", but his experiences with mental illness "defined his life".
Justice Porter told the jury it must keep calm and unemotional during the graphic trial, and it would be required to view images of the bodies.
"This can shock and distress people," Justice Porter said.
The jury was played a triple-0 call made by the accused in the early hours of the morning on December 19, 2012.
The accused said to the operator: "I've got two bodies here" and "I've killed two people".
Ms Mason said it would be up to the jury to determine if the accused was so unwell it would render him incapable of being guilty of murder.The trial continues today.