The boy stepped into the kitchen on the afternoon of October 30, 2018, to see his great-grandmother Bette Schulz Cohrs lying on the kitchen floor covered in blood.
"I tried to wake her up," the now-10-year-old told the Supreme Court in Melbourne on Tuesday.
"I was very scared. I didn't know what to do."
Cohrs called police as he left the murder scene, reporting both his 81-year-old mother's death and the fact the boy was home alone.
A jury rejected Paul Cohrs' claim he was not guilty of murder due to mental impairment. (Con Chronis/AAP PHOTOS)
He then left Victoria and drove 110km over the border into NSW, where he had killed his brother Raymond hours earlier.
At his May trial, Cohrs claimed he was not guilty of his mother's murder because he was mentally impaired.
But a Supreme Court jury rejected that theory, instead finding him guilty after only three hours of deliberations.
He did not face a separate murder charge in Victoria for his brother Raymond's death as it happened in a different jurisdiction.
Jurors were told the relationship between Cohrs and Raymond became strained in 2012 as they had conflicting ideas over the family business.
With the support of his mother, Raymond decided in 2018 to get appraisals on a number of properties connected to the business.
That included a Lake Victoria Station property in NSW that Cohrs and his wife were living in.
On October 30, 2018, Raymond and a real estate agent went to the property for the evaluation and were met at the gate by Cohrs, who called his brother a "liar and a thief".
The men drove to the shearing shed and, once Raymond was out of the vehicle, Cohrs grabbed his shotgun and fired two rounds into him.
He approached Raymond who was lying on the ground, asked him what it was like to feel pain, before shooting him again in the head and chest.
Cohrs told the real estate agent he wasn't going to hurt him and left him handcuffed in the shed before driving to his mother's Red Cliffs house, near Mildura.
He entered the kitchen and fired a single shot into her chest as she was standing.
Cohrs left the scene and drove back to Lake Victoria Station, where he freed the agent and tried to kill himself.
Police arrived and arrested Cohrs, who told officers jail was his worst nightmare.
Six victim impact statements were read to the Supreme Court on Tuesday as Cohrs faced a pre-sentencing hearing.
He stared straight ahead as his daughter Kristy called him an "evil man" who had taken away her soulmate - her grandmother Bettie.
"We were robbed of time with our nanna because of Paul's selfishness and greed," she told the court.
"Paul ripped out a huge part of my heart that day that I can never get back."
Defence barrister Lucien Richter said although the jury found Cohrs was not mentally impaired, his client still had a diagnosed delusional disorder.
Cohrs killed his mother and brother while experiencing intense anger and hatred towards them, Mr Richter said as he asked Justice Lesley Taylor to take the disorder into account when considering his sentence.
But prosecutor Melissa Mahady said it was a serious case of murder against a vulnerable woman, regardless of any mental illness.
The fact a four-year-old boy was present was also an aggravating feature, she said.
Justice Taylor will sentence Cohrs at a later date.
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