The prisoner who bashed Carl Williams to death says he threw his life away to protect his family from the man he killed.
Matthew Charles Johnson told the jury in his murder trial he was aware when he decided to kill Williams that he would spend a long time in jail.
"I knew I would spend the next 30 to 40 years in jail," Johnson told the Victorian Supreme Court on Tuesday.
"I was throwing my life away to keep my family safe."
Johnson has told the court Williams tried to intimidate him and belittle him when they moved in together in a high-security unit at Barwon Prison.
The jury has heard another prisoner tipped him off that Williams planned to kill him by bashing him with pool balls in a sock.
But the combination of a code of silence among prisoners and a fear of Williams' reach beyond the jail prevented him from reporting the threat or from seeking to be moved to another unit.
Johnson said he took the threat seriously because he knew Williams had killed a number of people.
"He was a killer," he said.
"If you're not going to take a threat from him seriously, there's something wrong with you."
The tip that Williams intended to kill him came from the third member of the prison unit, Tommy Ivanovic, a man Johnson described as "like a brother" to Williams.
Johnson told the jury Ivanovic warned him on April 18, 2010 that Williams was planning to attack him with pool balls.
He said Ivanovic told him he had walked in on Williams in his cell stretching a sock.
"He told me Carl said he was going to put pool balls in it and get me when I was eating," he said.
As a result, Johnson decided to get in first, taking the seat stem from a prison exercise bike and bashing Williams to death.
"I decided I'd get the bike stem and get him in the morning while he was having breakfast or reading the paper," he said.
Johnson, 38, said he was surprised prison staff hadn't spotted the attack on security cameras and arrived in the unit before he'd finished the attack.
"In my mind I thought the officers would have got there before I finished hitting him," Johnson calmly told the jury.
In response to questioning from his counsel Bill Stuart, Johnson said he wouldn't do anything different if confronted with the same situation again.
Johnson admits killing Williams but has pleaded not guilty to murder, saying he acted in self defence.
The trial before Justice Lex Lasry is continuing.
