An explanation that a teenager who stabbed an Auckland shopkeeper to death was defending himself has been slammed by the Crown.

Prosecutor Kieran Raftery told the jury during his closing in the High Court at Auckland that the lawyers of a 14-year-old murder-accused would argue that point when they closed their case tomorrow.

"You can be forgiven for being a bit surprised because the person who's defending himself is Mr Kumar," Mr Raftery said.

"The self-defence notion is utter rubbish."

The boy's 13-year-old co-accused is charged with the manslaughter of Arun Kumar after the incident at the Henderson dairy just over a year ago.

The pair, who have name suppression, originally faced additional counts of assault with intent to rob but Mr Raftery withdrew those charges this afternoon.

"It's not because there wasn't an attempt at a robbery but that's not what this case is about. It's not about stealing something from the dairy, it's about Mr Kumar being killed by those young men," he said.

CCTV footage from inside the dairy has been played to the jury repeatedly from various of angles and in slow motion.

And Mr Raftery this afternoon pointed out each allegedly aggressive manoeuvre, particularly by the knife-wielding older defendant.

"One of the things you'll have learned over the course of this trial is how long or how little time it took to commit a murder; how little time it took to kill someone," he said.

"The incident lasts approximately half a minute and Mr Kumar's life didn't last for many more minutes after that."

There was also evidence about the 13-year-old's upbringing during which his father took him to gang pads where he witnessed things a child should not.

"He may well be a reflection of his upbringing . . . but as tragic as their upbringing might be, they still did what they did," Mr Raftery said.

"Both of them may have difficult, tragic backgrounds, which have made them the criminals they are."

The Crown rejected any notion that the armed robbery was impromptu or that the boys did not have an idea about what might happen in the shop.

"They were not there for bottles of pop, they were there for money," Mr Raftery said.

"He had that knife at the ready."

The defence had previously called witnesses who detailed head injuries suffered by the 14-year-old and the affect it had, but the prosecutor was sceptical about that impact.

"The details [of the robbery] weren't planned out to their logical conclusion and you wouldn't expect that of 12-year-olds, whether they've got brain damage or not," he said.

Whether the boys considered the consequences of their actions or not, Mr Raftery said it did not matter.

"That's true of so many offenders, whatever age they might be," he said.

The Crown will finish its closing today and defence counsel will close their respective cases tomorrow.