TORONTO - A teenage girl with a "capacity for manipulation" and a "lack of empathy" who waged a relentless campaign to have her 14-year-old perceived romantic rival killed, was sentenced Tuesday to life in prison for the "malevolent" crime.

Previously identified only as M.T. because she was just shy of her 16th birthday at the time of the murder, her adult sentence for first-degree murder means Melissa Todorovic can now be named.

At her behest, Todorovic's then 17-year-old boyfriend lured Stefanie Rengel from her home on New Year's Day 2008. The boyfriend, known only as D.B., has pleaded guilty to stabbing Rengel six times and leaving her to die in a snowbank.

Ontario Superior Court Justice Ian Nordheimer said it was a "horrific" crime committed "for reasons that are still unfathomable," and that the facts of the case are unprecedented.

"One can only hope that they remain so given how truly malevolent they are in their content," he said in his decision.

"The concept that teenage angst becomes a driving force for murder leaves one both shocked by, and incredulous at, its manifestation."

Todorovic viewed Rengel, who had been in a brief and non-sexual relationship with D.B. when she was 12, as her rival and over the course of Todorovic and D.B.'s "stormy" relationship, Rengel grew for Todorovic "from an irritation to an obsession," Nordheimer said.

Todorovic "engaged in an unrelenting campaign over many months to cause the death of a 14-year-old girl that she had never met," he said.

She placed "relentless pressure" on D.B. through instant messages, text messages and Facebook messages threatening to withhold sex from him if he didn't kill Rengel.

D.B. earlier pleaded guilty to first-degree murder and his sentencing hearing begins in September.

Defence lawyer Marshall Sack had maintained that Todorovic was less culpable than D.B. because he was the one who actually stabbed Rengel, but Nordheimer disagreed.

"Put simply, the puppet master is not less blameworthy than the puppet," he told the court.

"Indeed, I would suggest that the master is more culpable since he or she puts the wheels in motion and then stands back under a facade of disassociation while the scheme they have created unfolds."

In handing down the adult sentence, Nordheimer set Todorovic's parole eligibility at seven years - the maximum for someone under 16 at the time of the offence.

Members of both families cried after the sentence was read, while after hearing the words "adult sentence" a hint of a smile crept across the face of Rengel's 13-year-old brother Ian.

Outside court he faced a throng of cameras to read a statement on behalf of his family.

"We are relieved at the adult sentence and we appreciate the difficult decision that Judge Nordheimer faced," said the boy, with his parents Patricia Hung and Adolfo Rengel close behind him.

"Melissa Todorovic is a disturbed individual who needs all the help that our system has to offer," Ian Rengel said.

"We pray that she benefits from the services now available to her and can grow to heal and become a balanced and rational member of society."

One pre-sentencing psychiatric report "notes Melissa's capacity for manipulation and her lack of empathy," Nordheimer said, adding that the same psychiatrist suggested if Todorovic re-offends potential targets include partners and "possibly" children.

The evidence suggests a "frightening" character flaw, Nordheimer said. He said he favoured an adult sentence because of the greater potential for monitoring her.

Todorovic would be completely free and unmonitored after serving a youth sentence of six years in custody and four under community supervision, whereas even if she is paroled after seven years in prison under an adult sentence the National Parole Board could asses her reintegration into society, Nordheimer said.

"Someone must be charged with the task of at least making an effort to follow Melissa's progress so that if a risk should again present itself there may be some hope of interrupting its path," he said.

Sack suggested Nordheimer erred in several areas and said he will be recommending an appeal. Todorovic, he said, is "devastated" by the sentence.

"You have to understand that she is only 17," Sack said.

"She's been in custody for 18 months and this has been for her an ordeal and yes, I understand that it's nothing like the ordeal that the Rengel family is suffering."

Nordheimer offered a few words to the Rengel family before delivering his sentencing.

"To the degree that it is possible for any stranger to do so, I understand and I sympathize with the loss that you have experienced and the pain and the sorrow and the anger that you feel," he said.

"Nothing the justice system can do, or might ever do, could ever possibly right such a terrible wrong."