Darren James Lawler, 37, pleaded guilty to Paul Shannon Andrews' manslaughter on July 27, 2018, after Mr Andrews, 41, confronted Lawler outside his Mango Hill home.

Drunk, drug-affected and suspicious his partner, Tiffany Baker, was having an affair after she walked out in him earlier in the day, Mr Andrews hid in the darkness behind trees waiting for Lawler, the Brisbane Supreme Court heard.

"What the f*** is my missus doing here?" prosecutor Chris Cook told the court Mr Andrews bellowed when Lawler finally emerged from his townhouse with Ms Baker.

"Mr Andrews was enraged over the belief Lawler and Ms Baker had been having sex."

"(His) suspicions were, of course, correct and his investigations left him playing the ultimate price."

Armed with a vodka bottle and 30cm-long torch, Mr Andrews struck the first blow, hitting Lawler in the head before being wrestled into a headlock.

Lawler then pulled out a military-style knife, saying "I'll stab you c***" and "forcefully" slashing Mr Andrews five times in the chest, neck, back and skull, penetrating half a centimetre into his head.

Two wounds near his heart were found to be fatal after Mr Andrews bled to death. The first perforated his left lung, the second severed cartilage.

"If he didn't have the knife, it's extremely unlikely Mr Andrews wouldn't be dead," Mr Cook said.

Police initially charged a blood-covered Lawler, who fled back inside his home when Mr Andrews stood up after being knifed, with murder.

But on Thursday he pleaded guilty to the lesser charge of manslaughter, which the Crown accepted.

Defence lawyer Tim Ryan tried to argue Lawler acted in self-defence but Justice Peter Davies was having none of it.

"If somebody is on top of someone and delivering blows with a military-style knife to his chest, such as to create a wound extending toward his heart, surely that is intent to kill?" he said.

Mr Andrew's mother, Yvonne Andrews, told the court her son's death haunted her family and she was heartbroken his four children no longer had a father.

"I have constant nightmares about the brutality of Paul's death," she said reading her victim impact statement to the court.

"Now there is nothing but isolation and extreme grief for the entire family."

In sentencing, Justice Davies said Lawler's "extremely violent" attack using a "substantial weapon" had escalated "a bit of a punch up to five incised wounds including blows to the man's chest and head".

"The production of the knife made what might have been, up to that point, a fair fight certainly an unfair one," he said.

Lawler was sentenced to eight years in prison with a serious violence offence declaration. He will be eligible for parole after serving 80 per cent of the term.

Australian Associated Press