Jordan Nelson's boyish looks belie his brutal crime.
He was aged just 13 years and 89 days when he shot his carer in the back of the head with a .22 calibre rifle.
His crime makes him possibly New Zealand's youngest murderer.
The Nelson Mail reports that Nelson pleaded guilty in the High Court at New Plymouth to the murder of Rosemaree Kurth, 50, earlier this year.
Nelson was due to stand trial later this month before changing his plea.
Crown solicitor Cherie Clarke said Nelson thought of Kerry Lock, who was in a relationship with Ms Kurth, as his grandfather and the three had lived at the address since September 2010, the Mail reported.
The court was told that about a week before the murder, Nelson was annoyed with Ms Kurth for removing a Freeview telvision receiver from his room.
It appears Nelson planned the murder, getting guns and ammunition ready while Mr Lock was walking the dogs.
He carried out the murder later in the day when he was again alone in the house with Ms Kurth.
Mr Lock arrived home and discovered the murder.
"The bedroom door was semi-closed. He opened the door and found Ms Kurth lying on her back with blood coming out of her ears and mouth," prosecutor Cherie Clarke said.
Nelson was arrested and when interviewed by police claimed to have no memory of the incident.
"He maintained he was going to the police in Waitara because he felt like he had done something wrong but didn't know what it was," Miss Clarke said.
Nelson later admitted to police Ms Kurth had died because he had shot her, Miss Clarke said.