Stefanie Rengel was found stabbed six times on a snowbank near her parents' Toronto home on New Year's Day, 2008. (Courtesy of the Toronto Police Service)

The Toronto teen accused of planning the murder of Stefanie Rengel in 2008 was only trying to keep up with her older boyfriend and never intended for him to stab her perceived rival, the girl's defence lawyer told her first-degree murder trial.

Defence lawyers for the accused teen, who can only be identified as M.T. because she was a minor at the time of the killing, presented their final arguments in a Toronto courtroom on Tuesday. The judge is expected to give final instructions to the jury on Wednesday morning, before jurors begin deliberating.

Crown prosecutor Robin Flumerfelt alleges M.T. masterminded Rengel's stabbing on Jan. 1, 2008, outside her parents' home in the district of East York.

On Monday, Flumerfelt told the jury that Rengel, 14, was the neighbourhood babysitter who taught Sunday school and loved the arts.

The only reason she was killed, the Crown argued, was because she was unwittingly drawn into the sights of the accused teen's hateful obsession.

Flumerfelt cited months of text messages and MSN chat logs, in which he alleged M.T. used sexual blackmail to spur her then 17-year-old boyfriend into killing Rengel.

Even on the day she knew he was going to stab Rengel, Flumerfelt alleged, M.T. refused to call it off.

In a police interview, M.T. was asked: "Why didn't you do something? Why didn't you call 911?"

Her answer: "I didn't think I had to."

Online chat excerpts taken out of context: teen's lawyer

But in his remarks before the jury, defence lawyer Marshall Sack argued Monday it was the boyfriend, not his client, who was the one doing the manipulating, as well as the one who first brought up killing Rengel.

Sack argued it was the boyfriend's "warped and convoluted brain" that misinterpreted M.T.'s words and used them to justify killing Rengel.

He told the jury the accused's boyfriend was "violent," "sexually aggressive" and had "no idea of the control his testosterone has over him."

Sack said segments of transcripts of the couple's chat sessions provided by the Crown lack the body language of a real conversation and have been taken out of context.

He cited one excerpt not mentioned by Flumerfelt, in which his client tells her boyfriend that revenge on Rengel for spreading rumours about the accused was "so three months ago," which Sack suggested meant the boy should forget about it.

Sack said M.T.'s angry words were evidence only of a teenage girl venting her frustrations in a private conversation with someone two years older than her.

The messages, he contended, didn't amount to a murder plan, but only illustrate the human dynamic between two adolescents who lack life experience.

Since the jury could never detect sincerity through the teens' demeanour in the online chat sessions, Sack said, they could never truly interpret the couple's intentions.

The girl's one-time boyfriend, who also cannot be identified, is slated to stand trial for first-degree murder in the fall.