Supreme Court Justice Elizabeth Hollingworth said on Wednesday that Michael O'Neill, known as 'The Talented Mr Ripley' because of the way he constantly lied,  had been in a "fragile psychological state" when he killed his partner of 16 years, The Age reports.

Justice Hollingworth accepted O'Neill, who stood in the dock with his head bowed as he was being sentenced, had murdered Mr Rattle in the heat of the moment without any forethought after an argument.

In sentencing O'Neill to 18 years' jail with a non-parole period of 13 years, the judge said Mr Rattle had often called him lazy, a parasite and threatened to send him back to where he came from.

Justice Hollingworth said Mr Rattle treated O'Neill as an office boy and not his partner but his behaviour in no way justified his murder.

"I accept you are genuinely remorseful for what you have done. There is plenty if evidence that you are devastated," the judge told O'Neill.

O'Neill, 48, who suffers from a dependent personality disorder, hit Mr Rattle over the head with a saucepan at the couple's South Yarra apartment at 6am on December 4, 2013, before strangling him with a dog lead as he begged for his life.

O'Neill claimed he snapped after Mr Rattle, 53, called him a "frigid bitch" for refusing to have sex with him that morning.

He placed Mr Rattle's body inside a large furniture bag and left it on their bed for the rest of the week as he went about his normal routine before returning on Sunday night, December 8, to set fire to the apartment.

Chief Crown prosecutor Gavin Silbert, SC, said O'Neill had killed Mr Rattle to try to cover up his incompetence which had been threatening to bankrupt the interior designer's business.

O'Neill had been responsible for accounts, quoting, invoicing and ordering stock for clients.

But defence barrister Ruth Shann said O'Neill, who spent more than five hours lying to police about what had happened before confessing to the murder, was in a rage because he felt so angry and demeaned by the way Mr Rattle had been treating him.

"It was a loving relationship but also a deeply imbalanced one," Ms Shann said. "Stuart liked to control and Michael liked to be controlled."

O'Neill, who came from a working-class family in Ireland before emigrating to Australia when he was four, had been a sad, inadequate and deeply dysfunctional man who did his best to create a persona of being one half of the perfect couple.

"The image and the brand of Stuart Rattle was important to both of them," Ms Shann said. "Stuart picked the clothes and directed how they would look and behave.

"They were good looking, had a social clique from the art world and Stuart was sought after by wealthy and connected people.  

"Even when the business was struggling, they flew first class, they bought the best wines, they bought the best things for their houses, they lived a first-class lifestyle."

Ms Shann said the problem was O'Neill had never come to terms with his sexuality and was ashamed of being gay.

"The real narrative here does not reveal a cunning, violent man, but a limited and damaged figure who killed the man that he deeply loved and to whom he was intensely attached and dependent, and then spent five days pretending it hadn't happened.  It wasn't a sophisticated cover up, but it was part of an ongoing pattern of repression and avoidance."

Ms Shann said one troubling aspect of the relationship involved Mr Rattle's demeaning and belittling of O'Neill in public and in private.  "The effect of this conflict, which occurred particularly when Michael did not acquiesce to Stuart's needs, intensified over the years.

"There was also an element of it which increasingly centred around sex, an area of ongoing difficulty and shame for Michael.  

"He was constantly in fear of losing Stuart, who was essentially the anchor to his life and had been for so long."

O'Neill told lies about anything that made him feel inadequate, including his background.

"He told people he went to university; he went to a posh private school in Melbourne; his parents were divorced; he wasn't Catholic.

"He said things because at the core he felt like the real situation was not good enough and in the 12 months or so before the death, the pressures of the business, the lifestyle they were living, the conflict between the two of them was increasing its effect on him in terms of resentment. Things were getting out of control."