A lorry driver had sarcastically remonstrated with another motorist seconds before he killed a boy, 3, on a pedestrian crossing, a court heard.
Dean Phoenix had made a clapping gesture moments before hitting Jaiden Mangan in Wareham, Dorset, prosecutors said.
He wept as he told Bournemouth Crown Court he "made a mistake" by not checking the crossing lights.
Mr Phoenix, 44, denies causing death by dangerous driving, in March 2018.
The defendant, of Cibbons Road, Chineham, near Basingstoke, earlier pleaded guilty to the lesser charge of causing death by careless driving.
He told the jury: "I just went to work that morning - I didn't go to hurt him."
The court heard Jaiden, who would have turned four the following day, was riding a balance bike and accompanied by his mother and his sister when he was struck by the Sainsbury's lorry driven by Mr Phoenix.
He told the court he had been trying to manoeuvre around the car, which was parked illegally on zig-zag lines, while the lights on the crossing were green.
Seconds before the crash, he had pulled forward to allow a bus to pass around the back of his lorry and assumed the lights were still green.
"I gave a quick check," he said. "I didn't see no-one."
The court heard his view of the traffic lights was obscured but the defendant admitted he could have seen them if he had moved in his cab.
"I was just observing what was in front of me," he said. "I could have [checked]. That's why I pleaded guilty to death by careless [driving] - it was a mistake."
Mr Phoenix said he would not have moved the lorry on to the pedestrian crossing if he thought there was a chance the lights were red.
Stuart Ellacott, prosecuting, said Mr Phoenix's clapping was recorded on a taxi's dashcam.
"He's frustrated, annoyed, sarcastic," said Mr Ellacott.
"He did not perform, I suggest, any of those basic checks that you would expect on a pedestrian crossing."
Paul Rogers, defending, asked the jury to consider whether it was fair to allege the crash happened "all because of frustration".
Mr Rogers said the car blocking the crossing "created a serious hazard in a sensitive area".
The trial continues.