Sex was the reward and obsessive jealousy was the motive for a young man's murderous actions on New Year's Day 2008 when he lured 14-year-old Stefanie Rengel outside her family's home, stabbed her six times and left her dying in the snow.

On Thursday, the now 19-year-old man sobbed after he pleaded guilty to first-degree murder in Rengel's killing.

The plea from the man, who can only be called D.B. because he was days shy of his 18th birthday at the time of the murder, comes less than one month after his former girlfriend was convicted in what prosecutors called a plot driven by her jealousy and orchestrated through online and text messages.

His girlfriend, then 15, was highly-manipulative and grew increasingly obsessed with the idea that Rengel was trying to steal her boyfriend, someone Rengel had dated briefly, according to an agreed statement of facts presented in court.

For months, the man and his girlfriend, known as M.T., exchanged thousands of text and instant messages, some of which discussed Rengel's murder.

A particularly chilling instant-message exchange in October 2007 talked of wanting Rengel dead.

The boyfriend, wrote: "What about Stef," according to transcripts filed with the court.

"Bang, bang," his girlfriend responded.

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

She replied: "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."

It didn't end there.

"Following that day, there were numerous MSN chats between them discussing murder," the agreed statement of facts reads.

On Jan. 1, 2008, the 15-year-old girl taunted her boyfriend with threats of having sex with someone else if he didn't comply with her demands.

Armed with a large kitchen knife, the man went to Rengel's house and lured her outside with a cellphone call. He stabbed her six times and left her to die in the snow just steps from the home of her parents, both Toronto police officers.

He ditched the knife in a yard and hid his jacket with the help of a friend before hopping a cab to his girlfriend's parents' house.

"Once inside, he told her what he had done, and they had sex, which was (his) reward for killing Stefanie," the statement reads.

Police arrested him later that night.

He was scheduled to stand trial on first-degree murder this fall, months after the trial of his one-time girlfriend. A jury deliberated for three days before finding the now 17-year-old girl guilty of first-degree murder on March 20.

After the man's surprise guilty plea Thursday, his lawyer said the young man hoped to spare the family the pain of a trial by taking responsibility for his actions.

"My client is incredibly remorseful for what he's done," Heather McArthur said outside of court.

"He knows that there's nothing he can do to change it. But by taking this step, at least he hopes to save the Rengel family further pain."

Rengel's family appeared to take some comfort in the plea.

"Although nothing will bring Stefanie back, we are relieved that both of her killers have now been convicted of first-degree murder," the family said in a statement read by Crown attorney Robin Flumerfelt.

The sentencing process for the 19-year-old man is schedule to start April 17.

Prosecutors will seek an adult sentence.

The Crown will also ask for an adult sentence for the convicted girl.

While the girl was not accused of actually ending Rengel's life, prosecutors relied on a well-established legal principle that counselling others to kill makes a person guilty of murder.

For a youth under 16 at the time of the murder and sentenced as an adult, first-degree murder carries a life sentence with no chance of parole for five to seven years.

For a youth 16 or 17 sentenced as an adult, the conviction carries a life sentence with no chance of parole for 10 years.

For a youth offender, the sentence is a maximum of 10 years, which includes four years served in the community.