The jury in a murder trial in Launceston has been told the alleged killer was incensed by his ex-partner's new relationship.
52 year old Michael Vernon Lowe of Waverley has pleaded not guilty to killing Darren John Booth at a house in Kings Meadows in May last year.
In her opening address to the jury in the Supreme Court in Launceston, prosecutor Linda Mason said Lowe went to the house in the early hours of the morning to see his ex-partner, Crystal Wells.
Ms Mason said the accused became enraged when he saw Mr Booth and produced a knife which he used to stab the 37-year-old three times in the chest after a violent struggle.
In his opening address, defence lawyer Greg Richardson told jurors they must consider the possibility that Lowe's actions were motivated by self-defence and take into account all of the circumstances leading up to Mr Booth's death.
A friend of Lowe and Ms Wells, Kristy Garwood, took the stand on the first day of the trial.
She told the court during a prior visit to her house the couple had argued about whether Ms Wells was having relationships with other men and Lowe produced a pocket knife and said he should kill himself.
Ms Garwood said on the morning of Mr Booth's death, Ms Wells came to her house and told her Lowe had stabbed him.
The courtroom was shown a police video of the scene, including vision of Mr Booth's body and his blood on the floors and walls of the house.
It prompted a man to swear at Lowe from the public gallery and he was told to leave the court, while a member of the victim's family became upset and walked out.
Another woman who was at the house at the time of the alleged murder told the court she had appealed to the two men to stop the fight.
She said a short time after she left the room she heard Mr Booth yelling that he had been stabbed and could not breathe.
The woman said Lowe apologised and left.
The trial is expected to run for at least a week.