When he picked up the stem of an exercise bike in April last year and delivered eight skull-crushing blows to the head of Carl Williams, he also became the man who murdered one of Australia's most notorious killers.
Inside prison he can now live up to his moniker.
As he sentenced Johnson on Thursday, Justice Lasry asked a question that many have asked since the killing: just how did Williams, a man who was assisting police, come to share a cell with Johnson, a man who hated police informers?
"How the prison authorities permitted that to happen is beyond me," the judge said.
Lasry had put it even more strongly at an earlier hearing, describing the decision to place the pair together as "breathtaking" and "amazing".
With Johnson's conviction and life sentence, the most obvious legal inquiry is out of the way.
Focus will now turn to a number of inquiries examining just how Williams could have been killed in Barwon Prison's Acacia Unit, the highest-security unit at Victoria's highest-security jail.
The day after Johnson's guilty verdict in September, Victorian Premier Ted Baillieu said a judicial inquiry might be necessary to investigate Williams' death.
"Clearly there was a failure at Barwon Prison," Baillieu said.
"It had dramatic consequences."
They are consequences Williams should have foreseen. He broke the underworld code that he once lived by. What happens to people who assist police should have been evident to him from the very case for which Williams broke the code.
Williams was providing information to police about the 2004 murder of a police informer.
If Williams thought he would be safe within the walls of Victoria's highest-security prison unit, he was wrong.
For starters he should have chosen one of the two people with whom he shared his unit more carefully. Johnson had form for bashing another man who had assisted police.
Twelve years earlier Johnson and several other prisoners bashed triple murderer Gregory John Brazel almost to death. One of the weapons used in the attack was the stem of an exercise bike seat.
Williams and Johnson had been friends for about 10 years, but Johnson was a man who lived by the underworld code.
Williams, who was in prison for life with a 35-year non-parole period, sold out when police dangled a very attractive carrot in front of him.
It included the payment of school fees for his daughter Dhakota and a A$750,000 ($986,000) tax bill owed by his father George and permission to apply for a reward.
Williams was also negotiating a non-financial deal with police, hoping that somewhere in the vicinity of five to 10 years could be cut from his sentence.
In return he was assisting police with their investigations into the 2004 murder of police informer Terrence Hodson and his wife Christine, who were gunned down at their Kew home.
In January 2009, Williams was taken away from prison for several days to talk to police about the murder. Williams tried to make it clear to Johnson he was only talking to investigators about corrupt police and not other prisoners.
Fearing how Johnson would react, Williams gave him regular updates about his discussions with police.
Copies of information Williams provided to police were downloaded onto Johnson's computer about six weeks before he killed Williams.
Johnson last accessed the information on his computer two days before the murder.
Lasry said Johnson killed Williams because he was assisting police.
"This was an appalling murder and is offending near the top of any notional range of offending," Lasry said. "It was a killing which appears to demonstrate your belief that you have some special entitlement to kill when you think it appropriate or your ego demands it according to some meaningless underworld prison code."
During his trial Johnson argued that he killed Williams in self-defence, an argument Lasry said was "fanciful".
Johnson also said he killed Williams because he feared for his family.
"The moment I made up me mind to kill Carl, I just assumed that the next 30 to 40 years I'm spending in jail," Johnson said during his trial.
"I was throwing my life away to keep my family safe, and to save it."
Perhaps other evidence from Johnson provides a more telling insight into the death of Carl Williams.
As Johnson put it, to him Williams was just another bare bum in the shower.
"He forgot he was in jail; he thought he was the boss of me," Johnson said.
