The judge in the Arun Kumar trial told the jury if they reject a teenager's explanation of self-defence they must find him guilty of murder or manslaughter.

During his summing up yesterday, Justice Lang took jurors through a "question trial" designed to help them systematically and logically reach their verdicts.

A 14-year-old boy is charged with the murder of Mr Kumar at the Railside Dairy in Henderson on June 10 last year.

His 13-year-old co-accused, who has stood trial alongside him at the High Court in Auckland for the past three weeks, is charged with manslaughter.

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The pair - who have name suppression - also faced counts of assault with intent to rob but the Crown withdrew those charges last week to focus on the more serious counts.

The judge said, in relation to the older boy, the jury would first have to consider what was in his head at the time of the attempted robbery.

Justice Lang said the Crown's case was clear: The murder accused used a deadly weapon to a vulnerable part of the victim's body in a bid to make his escape.

But during their closing addresses last week, defence lawyers asked jurors to look back in time to the factors that had shaped their clients.

Defence lawyer Maria Pecotic said the 14-year-old had suffered a severe head injury when he was hit by a car six years ago but was never given the aftercare he required.

The matter of "intent" was for the jury to decide if they rejected the notion of self-defence, Justice Lang said. He told the jury if they acquitted the older defendant, his co-accused was automatically cleared. To convict him of manslaughter, they must be sure he and the 14-year-old had formulated a plan and that the co-accused knew serious physical violence was a probable outcome.NZME