TORONTO - The 17-year-old girl convicted of using sexual blackmail to have her perceived teen rival, Stefanie Rengel, killed pushed back her long brown hair and glanced quickly around the courtroom before delivering a teary public apology Friday.

"Everyday I wish that I could go back in time and change everything I said and have Stefanie be alive with her family again," said the girl, known only as M.T.

Prosecutors successfully argued at trial that M.T. used the promise of sex to get her boyfriend, D.B., to stab Rengel six times outside her east-end Toronto home on New Year's Day 2008.

As she gasped between sobs, M.T. said she was sorry for everything she had said and done to contribute to Rengel's death.

"I want you to know that I take full responsibility for my part," she said. "I have learned many lessons from this tragedy."

The apology failed to move the Crown on its position that M.T. deserves an adult sentence for the 14-year-old girl's death. Rengel's family appeared unmoved as well -- her father shook his head from side to side as M.T. said she would never commit another offence.

"I believe treatment will help my disorders and that I will be able to contribute to society in a positive way when I finish my sentence," she said.

The now 19-year-old D.B. pleaded guilty to first-degree murder and has a sentencing hearing scheduled for September.

Prosecutors said M.T., who was found guilty by a jury, continued to minimize her role in the murder.

Her apology at the conclusion of the week-long sentencing hearing "fell well short of acknowledging responsibility," said Crown prosecutor Robin Flumerfelt.

"Her attitude has not changed."

A Toronto judge must now decide if M.T. will receive an adult or youth sentence. Ontario Superior Court Justice Ian Nordheimer said he will hand down his sentence on July 28.

The Crown wants an adult sentence with no chance of parole for seven years. The defence wants a youth sentence, which would mean 10 years in custody -- six served in jail and four served in the community under supervision.

Flumerfelt said M.T.'s heartless and deliberate behaviour before and after Rengel's death had to be taken into consideration.

"She embarked on her mission, her mission to put Stefanie Rengel, an innocent girl, in her grave," he said.

Flumerfelt said M.T's hatred was not directed at Rengel alone, but that she had and continued to harbour thoughts to harm people who said uncomplimentary things about her.

"This murder was not about Stefanie Rengel," he said. "This murder was about M.T. and her murderous heart."

Flumerfelt called M.T. "a purveyor of misery" who remained immune to the consequences of her crime. He said a youth sentence would fail to reflect the impact her crime had.

M.T.'s defence said she deserved a youth sentence because she was not physically involved in committing the murder.

"M.T. is a secondary participant. M.T. did not wield the knife," said defence lawyer Marshall Sack. "These are words not deeds."

Sack said M.T.'s conversations with D.B. prior to Rengel's stabbing were merely part of her obsessive and anxious personality, and did not amount to the calculated planning of a murder.

He added that her apparent lack of remorse were merely because she hadn't yet learnt how to feel such emotion.

"It's not that she's a femme fatale... or a bad seed," said Sack. "Her emotional and psychological development is... retarded."

"There is simply no evidence that M.T. was or is capable of actual physical violence."

The Crown countered this by saying M.T. had killed Rengel by imposing an order.

"D.B. was her weapon," said Flumerfelt. "She is a risk."

During the trial, the Crown entered into evidence more than 30,000 pages of instant messaging transcripts between the girl and her boyfriend, including conversations apparently detailing their plans for Rengel.

In one chilling instant-message exchange in October 2007, the accused appeared to suggest to her boyfriend she wanted Rengel dead.

The boyfriend, who once dated Rengel, writes: "What about Stef," according to transcripts filed with the court.

"Bang, bang," she replies.

He then writes: "I need a bang bang first ... I wanna bang you."

She writes: "I want her dead ... lol we've been through this ... If it takes more than a week then we're just going to be friends."