Alex Robert Smart admits plunging a knife into Tylor Bell's chest at the intersection in Gympie, north of Brisbane on September 1, 2019.

But he has denied committing murder.

Smart took action because of what occurred after the men stopped side-by-side in two vehicles at the lights and minutes earlier at a nearby shopping centre, defence barrister Jakub Lodziak told the Brisbane Supreme Court on Monday.

He said there was yelling before the stabbing and Mr Bell, 31, was either outside the ute he was driving, or at least had the door open.

During the earlier altercation, Mr Bell grabbed an implement from his ute but put it back when his father Gregory got into a fight with Smart.

Mr Lodziak said the Bells were considerably larger than Smart.

"He (Smart) had been assaulted, threatened, insulted, he knew what they were capable of and it was starting again," Mr Lodziak added.

"It was not an act of revenge - Mr Smart did what he did because he had a taste of what they were capable of."

Mr Lodziak said the Bells went "from zero to arming themselves and resorting to violence over God knows what - seemingly little or nothing - in the middle of the day in a busy shopping centre car park".

He argued Smart was minding his own business at the shops, while Mr Bell was aware of his presence and "wouldn't let go of whatever hang-up he had".

But prosecutor Dejana Kovac said Smart was angry after the shopping centre altercation and wanted to get even.

She argued the car in which Smart was a passenger deliberately pulled up next to the Bells' ute.

Smart lied to police about what occurred, including to two undercover officers put in a cell with him "to paint himself in the best light possible," Ms Kovac said.

She told the jury Smart was not acting defensively, but rather maliciously during a "brutal unsuspecting attack".

Witnesses said a verbal exchange at the shopping included Mr Bell calling Smart a "junkie" before Gregory Bell got out the ute, asking Smart: "Have you got a problem?"

Seeing Smart's closed fist, Gregory Bell punched him in the face.

Smart responded by kicking his legs, causing the older man to fall before quickly getting up.

The tussle was broken up by bystanders, but the men's vehicles ended up next to each other - Mr Lodziak says not deliberately - at the nearby lights.

During that altercation Tylor Bell was stabbed in the chest, while Smart's hand was cut.

Both men drove straight to the same hospital, but Tylor Bell went into cardiac arrest and died a week later.

The trial continues before Justice Peter Applegarth.

Australian Associated Press