Tylor Bell planned to see his five children after buying groceries with his father Gregory at a shopping centre in Gympie, north of Brisbane on September 1, 2019.

But he never made it home.

Alex Robert Smart has admitted stabbing Mr Bell at the light a few blocks from the supermarket.

But the 29-year-old pleaded not guilty to murder at the start of the Brisbane Supreme Court trial which defence barrister Jakub Lodziak said is not so much "a whodunnit".

The court has been told there was yelling between the Bells and Smart at the shopping centre entrance as the father and son were driving out the car park about 2.30pm.

Witnesses said the verbal exchange included Tylor Bell calling Smart a "junkie".

Gregory Bell said he got out the ute when his son abruptly stopped, asking Smart: "Have you got a problem?"

Smart replied: "Do you want a go?", Mr Bell told the court on Wednesday.

Seeing Smart had a closed fist, Mr Bell gave him a "left jab" in the face.

"I thought he was going to hit me," Mr Bell said.

A tussle between the men was broken up by bystanders, but at the nearby lights Mr Bell saw Smart in a car approaching slowly from behind.

Mr Bell said Smart was soon at the driver's door, preventing Tylor Bell getting out.

Mr Bell ran around the ute, trying to grab Smart who left in the other car.

Unaware his son had been stabbed Mr Bell said to follow the vehicle to get the number plate.

"He looked at me and said, 'I think I've been stabbed'," Mr Bell said.

He told the court he did not take much notice, but his son repeated: "I feel really sick, I think he stabbed me."

Mr Bell told his son to drive the few blocks to the hospital where they saw Smart in the emergency department and he told Smart he'd stabbed his son.

Mr Bell denied telling his son he would "flog" Smart if Tylor Bell stopped the ute at the centre.

Prosecutor Dejana Kovac earlier said Smart had approached the driver's door of the ute stabbing Tylor Bell twice, with one wound so deep it penetrated his heart.

Tylor Bell went into cardiac arrest at the hospital and died a week later.

Mr Lodziak said he would not challenge the suggestion Smart was holding the knife at the time the fatal wound was inflicted.

But he told the jury that questions about how and why it happened would be important to consider.

"Was it because Alex Smart actually intended to kill the deceased, or at least cause him some grievous bodily harm?

"Was it because Alex Smart was acting under a set of circumstances that meant what he did was not unlawful?"

The trial before Justice Peter Applegarth is expected to hear from about 20 witnesses.

Australian Associated Press