Michael Villalon, 29, is on trial in the NSW Supreme Court for the murder of Keith Allan Chaney, 69, in the southern Sydney suburb of Mascot on February 28, 2012.

Mr Villalon does not deny that he killed his mother's partner, but says that he should be found guilty of manslaughter rather than murder because he was affected by a mental condition that reduced his ability to understand what he was doing or to tell right from wrong.

In his opening address to the jury on Monday, Crown prosecutor Giles Tabuteau said Mr Villalon had inflicted a series of stab wounds to the older man's face, neck and torso with two knives, and then beat him over the head with a hammer.

After the killing, Mr Villalon allegedly told police: "I just exploded - I take full responsibility ... A couple of weeks ago, he hit me - he punched me in the face."

He later told police "the humiliation had to stop".

Mr Tabuteau said there had been growing animosity between Mr Villalon and Mr Chaney in the months after the young man and his sister moved into Mr Chaney's house on Gardiner's Road, Mascot, in 2011.

The two men allegedly argued on a regular basis over domestic matters, with Mr Villalon complaining that he was being nagged and picked on.

On one occasion, Mr Chaney allegedly became angry at the young man for taking nuts that were roasting in the oven.

"There was also an argument over a towel not being put back," Mr Tabuteau said.

"On another occasion, there was an argument at the dinner table over the accused not using a knife when he ate his meals. Mr Chaney also told Michael Villalon he was lazy and should get a job."

The pair also allegedly clashed over the fact that Mr Chaney watched a lot of pornographic movies, including child pornography - a crime he was briefly jailed for in 2009. He also allegedly attempted to kiss Mr Villalon's sister.

After the killing, Mr Villalon told police that, on the day of the killing, Mr Chaney had become angry at him for closing the bathroom door - something the older man had objected to strongly in the past.

The older man had then threatened him with a hammer, Mr Villalon said.

He admitted to then stabbing and beating the 69-year-old to death. But he now says he was affected by a mental disorder that impaired his ability to tell right from wrong.

The court heard that Mr Villalon had been depressed and complained about hearing voices before the killing, and had cut his wrists and taken sleeping pills and mild pain killers immediately afterwards.

However, assessments of the young man's mental state in the days after the attack suggested that he had been aware of what he was doing.

However, he then experienced a psychotic episode in jail.

"The central issue in this case will be the capacity of the accused to understand whether his acts were right or wrong, or to understand the events, because of an underlying condition," Mr Tabuteau said.

"If there was a substantial impairment, it is up to you to decide whether it is significant enough to reduce his responsibility from that of being guilty of murder to being guilty of manslaughter.

"The Crown says you would not find that is the case."

The trial continues.