Justice Natalie Adams found Warren Anthony Scott, 38, guilty of Edward "Eddie" Carter's manslaughter at his mother Jessie Scott's Bega home in April 2019.

It had been a "sustained attack on an unarmed man who caused him no harm," the judge said in her NSW Supreme Court verdict on Wednesday.

But Scott's longstanding diagnosis of schizophrenia was likely the cause of a psychosis he was unable to explain due to his intellectual impairment, she said.

It is believed Scott was drinking in a park daily for about two weeks leading up to the incident when his cousin Kevin Dixon picked him up.

The usually "happy laughing" jokester appeared very different on this day with a blank stare that "freaked" Mr Dixon out.

Mr Carter had been staying with Ms Scott who considered him a brother of more than 20 years, when Scott was dropped at his mother's Eden home.

The three were seen drinking together listening to music and "having a good time".

Suddenly Scott jumped up and retrieved a large knife from the kitchen and held it to his mother's throat, before he calmed down and put the weapon away.

But after he threatened her for a second time with a smaller steak knife Mr Carter intervened and his mother fled seeking help.

An attending officer said Scott drew the curtains at one stage to hide the hostage situation and continuously yelled "f*** off pigs go away," to their questions.

An "emotionless" Scott was later seen holding the blade right between his victim's eyes before thuds were heard travelling around the house.

After Mr Carter yelled he had been stabbed Scott could be heard saying he would watch him "bleed out".

Ms Scott earlier screamed at police "kick the door in and save my brother," before a gurgling noise prompted them to force entry inside.

Multiple eye witness accounts said Scott did not appear himself, one relative saying he acted "eerily calm" and his eyes looked "really really dark".

His mother had seen her son drunk countless times but never looking and sounding "possessed ... something evil you see in a movie, the way he looked the way he sounded just so evil".

Mr Carter and Scott had never shown any animosity towards one another.

An excuse thought up a year later by the aggressor about being owed money was dismissed.

While on a drinking spree Scott was known to stop taking his medication for schizophrenia, diabetes and epilepsy.

Justice Adams said Scott was unable to explain why he ever lunged at his mother, the person he typically sought protection from, and this was likely due to some kind of psychosis.

The Crown submitted that actions such as shutting the curtains and later saying "I'm going to jail for the rest of my life," showed some level of understanding.

But the judge pointed to "disturbing" actions of his bartering for cigarettes moments after the fatal stabbing.

"I do not consider a person just caught in the act of murdering someone would behave in this manner if thinking rationally," she said.

"I am satisfied the accused's capacity to understand events and judge whether his actions were right or wrong and to control himself was substantially impaired at the time of stabbing."

Scott will be sentenced at a later date.

Australian Associated Press