WA man murdered in 'revenge' for threesome

A former fly-in fly-out worker has admitted fatally stabbing a man who had a threesome with his girlfriend at a caravan park but claims he acted in self-defence, a Perth court has heard.

Ryan Christopher Hawke is on trial in the West Australian Supreme Court, accused of murdering Mark Koller, 50, at the Crystal Brook Caravan Park in Orange Grove in April last year.

In his opening address on Wednesday, prosecutor Sean O'Sullivan said it was not a case of self-defence because Hawke took a knife with him to the confrontation, stabbed the victim six times and did not stay to help.

"It was aggression based upon anger and a desire for revenge," Mr O'Sullivan said.

The court heard that when Hawke was away for work, his girlfriend became friendly with Mr Koller's partner and had a threesome with the couple.

When Hawke returned and learned about the sexual activity, he decided he needed to teach Mr Koller a lesson, so he armed himself with a knife in his sock and brought his girlfriend with him to confront the victim, Mr O'Sullivan said.

He said Mr Koller tried to defend himself with a gas bottle but was stabbed twice in the back and four times in the chest and abdomen, falling on top of Hawke.

Defence lawyer John Prior said his client had been told by his girlfriend that Mr Koller had a replica gun, so he took the knife with him for protection.

Mr Prior said Hawke was only expecting to have a fist fight and was acting in self-defence because Mr Koller attacked first and was armed with a gas bottle and screw driver.

"He didn't go there to kill him. He didn't go there to stab him. He went there for a confrontation," Mr Prior said.

The trial continues.