The jailmate of Carl Williams has today been sentenced to life behind bars for murdering the notorious gangland killer inside Barwon Prison.

In a packed Victorian Supreme Court, Justice Lex Lasry described Matthew Charles Johnson’s actions in murdering Williams on April 19 last year in the high-security Acacia unit they shared as "appalling".

"This was an appalling murder," he said. "It was a killing which appears to demonstrate your belief that you have some special entitlement to kill when you think it is appropriate, or your ego demands it according to some meaningless underworld prison code.

‘‘I am left with no choice but to punish what you have done with an appropriately heavy sentence."

He ordered Johnson, who showed no emotion in court, to serve a minimum of 32 years before becoming eligible for parole.

The 38-year-old had pleaded not guilty to the gangland figures murder.

Williams was serving time in the unit after he was sentenced to life with a minimum of 35 years for the murders of Michael Marshall (October 25, 2003), Mark Mallia (August 18, 2003), Jason Moran (June 21, 2003) Lewis Moran (March 31, 2004) and conspiracy to murder Mario Condello (June 2004), who was later gunned down in 2006.

Johnson admitted killing Williams by repeatedly striking him over the head with the metal stem of an exercise bike seat, but claimed he did so in an act of self defence after being told the previous afternoon by the unit’s only other inmate, Tommy Ivanovic, that Williams was going to kill him by hitting him over the head with pool balls in a sock. He described this as a ‘‘known prison weapon’’.

Johnson told the jury in his defence that when he attacked Williams, he ‘‘intended to try and kill him’’ and had struck him a further seven times as he lay motionless on the floor, after the initial blow knocked him off the chair he was sitting in, to ‘‘make sure he was dead’’.

But the jury rejected his defensive homicide claims, that is, that he was in a situation of ‘‘kill or be killed’’ and felt he had no alternative but to get Williams first.

Williams was co-operating with authorities about the unsolved murder of police informer Terence Hodson and his wife, and had implicated a former drug squad detective, Paul Dale, as the man who had requested and financed the hit on the couple.

In three statements Williams made to police, he claimed to have been approached by Dale to ‘‘get’’ Hodson to stop him from testifying against Dale about his alleged involvement in a drug burglary.

Williams also named the gunman who pulled the trigger after he claimed to have approached the veteran hitman about the job.

The hitman cannot be named for legal reasons.The jury heard that Johnson — who dubbed himself ‘‘The General’’ of a jail gang that called themselves Prisoners of War — despised ‘‘dogs’’, the name given to people who provided police information about others.

But the trial heard that Williams felt he was able to explain and reassure Johnson about why he was assisting them, and justified his actions because it was connected to police corruption.

Johnson claimed that Williams was ‘‘pulling the wool over the eyes of police’’ and was going to ‘‘shaft’’ them, so he said he didn’t really consider him a dog at all.

On the day of his death, a page one article in the Herald Sun detailed several benefits Williams was receiving from police for his co-operation.

They included having his daughter’s school fees paid and clearing a $750,000 debt Carl’s dad George Williams had with the tax department.

Williams spoke to his barrister over the phone that morning but did not mention any concerns about Johnson or Ivanovic.

The court heard it was common knowledge within the prison system that Williams was providing police with information, and that it had been widely known before the article was published that he was receiving benefits in return.

Johnson said he had some of Williams’s legal documents in his cell and on his computer, because that was nothing unusual for him, Ivanovic and Williams to share information with each other about up-coming court cases.

Graphic CCTV footage aired at the trial shows Johnson approaching Williams from behind as he read the newspaper in the day room at 12.48pm on April 19, 2010.

Johnson struck him to the right side of his head with the heavy metal seat pole, causing Williams to fall to the floor. He then struck him another seven times in the head, before dragging Williams’s limp body into his cell and closing the door behind him.

Ivanovic, who was in the day room at the time of the attack, watched on. Security vision shows Johnson and Ivanovic walking laps of the exercise yard and entering the day room a number of times before they approached a prison guard about 1.15pm to tell her she should press her panic alarm because Williams had ‘‘hit his head’’.

Several prison officers then entered the unit and located Williams in his cell, covered in blood and with extensive head injuries.

He was dragged into the day room as guards and medical staff performed CPR for more than 30 minutes. Williams could not be revived.

Johnson was interviewed by homicide detectives later that night. He remained tight-lipped and answered ‘‘no comment’’ to most of the questions, but then told them: ‘‘I acted alone’’.

He told his trial that from the moment Ivanovic moved into the Acacia unit with him and Williams, his relationship with Williams — a man who the trial heard nicknamed himself ‘‘The Premier’’ because he said he ‘‘made all the hard decisions’’ — changed.

Johnson said Williams was always ‘‘flexing his muscles’’ and ‘‘hanging s---’’ on him and went out of his way to intimidate him in front of Ivanovic.Williams treated him ‘‘second-rate’’ and talked down to him and ‘‘pushed my buttons’’ whenever he had the opportunity, he said.

On one occasion, Johnson said he was inside his cell sitting at his computer one day, when Williams walked in and ran his fingers across Johnson’s throat.

‘‘He said: ‘that’s how easy it would be’,’’ Johnson said, adding that Williams then walked away laughing.

Johnson said didn’t think it was very funny. ‘‘I wanted to punch him but what could I do?’’

But Johnson said he strategically placed items in the prison unit in case he needed a weapon to attack Williams, if a fight ever broke out between them.

These included placing an exercise bike outside his jail cell and a sandwich maker on a table tennis table at the other end of the day room.

‘‘If me and Carl ever got in a fist fight, I knew in my heart I would have to kill him or my family would be,’’ Johnson said.

He said from the moment Ivanovic told him Williams was going to kill him, he knew he would have to get in first.

‘‘From the moment I’d made up my mind to kill Carl, I just assumed that I’d been spending the next 30 to 40 years in jail ... I was throwing my life away to keep my family safe,’’ Johnson told the court.

He told the jury that he thought the attack would have been witnessed by the guards tasked with monitoring the inmates via security cameras. He added he was surprised when no one responded straight away.

Johnson has more than 150 prior convictions, ranging from offences committed as a 15-year-old, to armed robberies and crimes committed in prison.

He was jailed last November for a minimum term of 13 years after pleading guilty to offences of armed robbery and aggravated burglary with another man that were committed in May, 2007.

Several months after his arrest in 2007, he was charged with the murder of 18-year-old Bryan Conyers, but was found not guilty in 2009 by a jury.

That verdict could not be reported because of the unresolved charges that were finalised last year. Johnson could not be identified last year when he received that long sentence because of the pending trial for murdering Williams.

Justice Lasry said that while he agreed that Johnson was a career criminal who had low prospects for rehabilitation, but added "anything’s possible".

"You impressed me as intelligent and articulate," the judge said. "Those qualities seem to me to have been wasted on the life you have lived."

Johnson said nothing as he got up and was ushered away by prison guards to begin his sentence.