TORONTO - Anything less than an adult sentence won't adequately protect the public from a young man who stabbed a 14-year-old girl to death in exchange for sex with his girlfriend, the Crown argues as the man appeals his sentence.

David Bagshaw was just four days shy of his 18th birthday when he lured Stefanie Rengel out of her Toronto home on New Year's Day 2008, stabbed her six times with a eight-inch kitchen knife and left her to die in a snowbank.

He pleaded guilty to first-degree murder and was sentenced to life in prison with no chance of parole for 10 years -- the maximum for a 17-year-old offender being sentenced as an adult.

Melissa Todorovic, his 15-year-old girlfriend at the time, was sentenced as an adult to life with no chance of parole for seven years -- the maximum adult sentence for someone her age.

Bagshaw is now appealing, arguing the maximum youth sentence of 10 years, in addition to his 21 months spent in pretrial custody, was more appropriate.

The Court of Appeal for Ontario is set to hear arguments Wednesday.

The Crown argues the gravity of the crime and the need to protect the public from Bagshaw mean only an adult sentence can hold him accountable.

"The circumstances of this offence shock the collective conscience of right-thinking persons," the Crown writes in court documents.

"It was a brutally violent murder committed upon an innocent girl who did absolutely nothing to incite feelings of malevolence in the appellant. He acted for the purpose of pleasing his girlfriend and ensuring her sexual favour."

Psychiatrists have reported Bagshaw is at a substantial risk to violently reoffend. He has a long history of anger issues, anti-social behaviour and aggression that continued even during his incarceration, the Crown said.

If Bagshaw had waited four days to kill Rengel he would have, as an adult, automatically been sentenced to life in prison with no chance of parole for 25 years.

Superior Court Justice Ian Nordheimer erred in granting Bagshaw credit for time served, his lawyer, Delmar Doucette, argues in material filed with the court. The judge was not obliged to do so, Doucette said.

If credit is granted it reduces the amount of time that can be served under a youth sentence, Doucette said, therefore leaving less time in which to become rehabilitated and necessitating an adult sentence.

A youth sentence is "capable of both promoting his rehabilitation and reintegration into society and of holding him, a reluctant offender who was extremely immature at the time of the offence, accountable to society."

He also suggested Nordheimer placed an impossible burden of proof on the defence by saying neither of the two psychiatrists who examined Bagshaw would say "with certainty" that he would engage in treatment.

Bagshaw had a history of non-compliance and the judge properly weighed it as a factor in assessing Bagshaw's prospects for rehabilitation, the Crown argues.

One psychiatrist said Bagshaw feels "no empathy in certain circumstances," the Crown noted.

"Unspeakable harm was caused by the appellant to Stefanie Rengel and her surviving family, as well as the broader community," the Crown writes.

"He robbed a happy and promising 14-year-old girl of her future. He took her life with great cruelty, leaving her to die alone in fear and pain on a dark, cold street."

Nordheimer found that Bagshaw and Todorovic had a sexual relationship "marked by mutual obsession and jealousy" that culminated in Bagshaw killing Rengel after months of pressure from Todorovic, who mistakenly saw Rengel as her rival.

Todorovic had been "hounding and manipulating" Bagshaw to kill Rengel for months, threatening to withhold sex, end the relationship, have sex with another boy that Bagshaw knew or kill herself unless he went through with it, the court heard.