KEY POINTS:
It seemed an ordinary summer school holidays weekend for 15-year-old Kori Trevithick.
On the Friday, the North Shore teenager took the bus to his girlfriend's place.
On the Sunday they went babysitting together.
And in between, Trevithick broke into the Birkdale home of 77-year-old widow Doreen Reed, took a knife from her kitchen drawer and stabbed her 25 times as she slept in her bed.
It was an inexplicable, random crime. Trevithick didn't know Mrs Reed to have a grudge against her, he wasn't confronted by her, and there was apparently no sexual motive.
He had no history of violence, no childhood difficulties or emotional problems. He had drunk alcohol on the night, but not a lot.
The question "why?" has puzzled his hard-working Glenfield parents, detectives and Mrs Reed's relatives alike.
Trevithick has offered no explanation since the January murder.
This week, the former Glenfield College student was sentenced to 14 years in jail after pleading guilty to the crime. He told police he blacked out when it happened and could not remember anything. His photograph cannot be shown because of a court order.
What is known is that Trevithick was staying the night at his girlfriend's parents' home. He argued with her in the early hours of Saturday, January 13, and left on her bike.
He pulled up about one kilometre away at Mrs Reed's house, turned the power off at the mains and broke into the house. He crept up on her in her bed and, just as she woke, began his frenzied attack, plunging the knife through her heart and brain.
Trevithick has said he chose the house at random, but the Weekend Herald understands a friend of his girlfriend lived nearby, so he may have had some knowledge of Mrs Reed.
He stole her Rover car and left it parked 50m down the road from his parents' house and resumed his weekend.
Police came looking for him during the murder inquiry and his mother took him in to the police station, believing he had breached bail conditions imposed after he appeared in the Youth Court on car theft and burglary charges.
His family are dumbfounded by the crime, and his mother cried uncontrollably in the High Court this week when the grisly details were revealed.
Trevithick is the youngest of four boys, and the family say his only complaint about family life was that he did not see his mother Arlene, a shift-worker, and father Kerry enough.
The family this week said he was "besotted" by his girlfriend, whom he had known since he was 8, when they met on a camp, and they spent a lot of time together.
The girlfriend - whom the Weekend Herald has agreed not to name - continues to visit him.
Court documents referred to the relationship as "demanding", and it is understood he had started stealing to keep up with his peers.
His family say the relationship was normal first love.
Although Trevithick had been skipping school, drinking, using cannabis and stealing for a year, the family say the murder was out of character. His application to his studies had been good up to high school.
The family have not tried to quiz Trevithick on the reasons he killed Mrs Reed for fear of aggravating a fragile mental state that has led to his being put on suicide watch in prison.
Court documents say Trevithick can be manipulative and is accustomed to getting his own way.
He has a tendency to close down emotionally and not deal with his problems, the documents say, and has a mental age younger than his years.
The family say that Trevithick has received no psychiatric therapy to help him explain why he committed the crime.
They worry about what will become of him during 14 years in prison, particularly when he reaches an adult jail.
Detective Senior Sergeant Kim Libby, who headed the investigation, has conceded to Mrs Reed's family and friends that police may never know why Trevithick killed her.
Mr Libby said that although Trevithick was on Youth Court bail when he killed Mrs Reed, he was not an out-of-control youth criminal.
"Of course he can explain why he did it," said Mr Libby. "He just won't."
Reed possibly a 'symbolic' victim says FBI profiler
Doreen Reed may have been a "symbolic victim" when Kori Trevithick killed her, says a former FBI criminal profiler and world expert in the murder of elderly people by young killers.
The sheer brutality of the Trevithick's crime in January prompted Mark Safarik to make contact with the police inquiry team after reading of it on nzherald.co.nz.
He retired last month, but in his 10 years with the FBI's behavioural analysis unit he studied hundreds of cases of elderly women who were raped and murdered.
He told the Weekend Herald the question of "why?" was one law enforcement agencies worldwide had asked him.
"Crimes like [Trevithick's] are symbolic crimes. Often there is no motive. We have to look to the offenders' desire for power and control as the subtext for the attack. They feel like they don't have it themselves so they seek out someone elderly and helpless."
Mr Safarik said although Trevithick did not have a sexual motive he fitted a profile of other juvenile homicide offenders.
He said the murder of elderly people was a "phenomenon" that has existed for decades and many involving smothering may have gone undetected.
Mr Safarik said extreme "overkill violence" like that Trevithick used was commonplace, with his research showing the younger the attacker the more violence was used.
The lack of impulse control that led to the violence was the same that caused him to lash out in the first place. "Look in the 48 hours before the crime and see if you can find a precipitating event. It doesn't take a lot: a parking ticket, a fight with the girlfriend or an argument with a family member."
