Broughton also acknowledged the "irreparable pain and loss he has caused to the victim and her family," the lawyer said.
Justice Graham Lang, who imposed a minimum non-parole period of 12-1/2 years, did not buy the convicted youth's version and based his sentencing on the prosecution's summary of facts, which had not been disputed.
Broughton's DNA was found on the dead woman and he was alone when security cameras filmed him nearby smashing windows and doors around Taupo-Nui-a-Tia College shortly before Ms Aim was felled from behind by a blow to the head.
"There was never a shred of evidence anyone else was involved," said the judge.
Broughton showed no emotion throughout the proceedings, sitting in the dock beside a prison guard, head bowed for much of the time.
Justice Lang said he put the issue of the mystery man to one side. Through his counsel, the prisoner had accepted he should be sentenced as a principal, not a party to murder.
"I am sentencing you and you alone."
Karen Aim had left a party about 2am to walk about 2km home. About the same time, Broughton - riding a distinctive bicycle - was smashing glass windows and doors around the college "with a considerable amount of force," the judge said.
That triggered a security alarm at 2.14am, by which time the teenager must have seen Ms Aim walking past. He had been in an "extremely aggressive frame of mind" that night but insisted he had not been under the effects of drugs or alcohol.
"We have no way of knowing if she ever became aware of your presence before she was attacked on the footpath only metres from her home," said Justice Lang.
As the badly injured woman lay on her back on the concrete, a further blow or blows fractured her skull and she suffered massive brain injuries, which resulted in death a short time later.
Broughton pulled up her skirt, deliberately ripped her underpants and hitched up her clothes so that she lay exposed.
"It can only have been done for your own satisfaction in some way. At the very least, it was a severe indignity, and you left her in that state," the judge said.
A police constable responding to the college vandalism alarm found the injured woman by chance minutes later, lying in a pool of blood and making a gurgling noise,
Justice Lang said Broughton went home with her handbag and digital camera. In a "cynical move" he erased all of Ms Aim's photographs of her travels and took pictures of his own family and home.
Outside the court, Mr Wilkinson-Smith said his client had been sentenced on the basis that he was the only offender.
"The judge has made that clear."
The non-parole term was a long time "but it could have been longer." No appeal was planned.
Ms Aim's parents and brother were in court for the sentencing before returning tomorrow to their home in the Orkney Islands.
Father Brian Aim said justice had been done and he held no grudges.
Rather than anger, he felt sorry for the youth who had killed his beloved daughter.
"He has taken away Karen's life, but he has also destroyed his own," Mr Aim said.
"If I go down the road of anger and revenge that would only destroy my life."
He had been asked for months what he thought the killer's sentence should be, when it was not an issue for him.
"It's too late, Karen is dead."
- NZPA
