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 the High Court at Auckland for the past three weeks, is charged with manslaughter.
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.
They may also have to decide whether he used reasonable force in the circumstances.
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.
Defense 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.
Neuropsychologist Dr Valerie McGinn gave evidence that the boy had a reduced window of opportunity to think things through when they became complex and would have been acting on impulse when inflicting the fatal wound.
The matter of "intent" was for the jury to decide if they rejected the notion of self-defence, Justice Lang said.
Earlier he told the jury feelings of sympathy and prejudice would arise "on many levels" for but that they must be avoided to focus on the facts.
"The most obvious [example] of course is that Mr Kumar was killed and Mrs Kumar left a widow. That was a tragic event and all of you would be inhuman if you didn't have sympathy for her," he said.
"There's no dispute [the 14-year-old] caused Mr Kumar's death and initially some of you might harbour prejudice against [him] because of that."
But the judge told the jury to focus on the "issues sheet" he had given them to avoid personal feelings.
He also warned them about a couple of witnesses.
Robert Watene - a transient Henderson man - said he had seen the boys sparring in the days leading up to the killing.
"That evidence is not going to assist you in determining what was in those young persons' minds at the critical times in the dairy," he said.
Another teenager had followed the defendants to the dairy on the morning of the incident and gave evidence of what was said before and after the alleged murder.
But Justice Lang said his testimony must be approached with "considerable care".
The teen lied to police during his first interview and defence counsel said he was still lying to minimise his role in what happened.
Justice Lang 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.
The jury has started deliberations.
