TORONTO - The public got its first glimpse Wednesday into the mind of a young man who stabbed 14-year-old Toronto girl Stefanie Rengel to death to placate his "manipulative" girlfriend's unfounded teenage jealousies.

The 19-year-old man's immaturity, impulsivity and deep-seated aggressive nature left him vulnerable to his "obsessive" and "unempathetic" girlfriend's relentless pressure to murder, a court-ordered psychiatrist told the man's sentencing hearing.

The man, who can only be identified as D.B. because he was just shy of his 18th birthday the night he killed Rengel, pleaded guilty earlier this year to the New Year's Day, 2008 first-degree murder.

His girlfriend at the time, Melissa Todorovic, now 17, was sentenced as an adult in July to life with no chance of parole for seven years for being what the judge called the "puppet master" in the murder.

Much was made at Todorovic's trial of her behaviour; she was described as a self-pitying girl with a lack of empathy and capacity for manipulation. But since D.B. pleaded guilty, he had no trial and the public learned little about his personality.

The psychiatrist's report presented Wednesday shed light on the interior life of the man who at his girlfriend's behest plunged a knife six times into a girl he bore no ill will toward.

Rengel would likely still be alive if it weren't for the unfortunate clashing of two obsessive personalities in a stormy relationship, Dr. Lisa Ramshaw surmised in her report. The pair constantly accused each other of cheating, broke up almost daily, and spent their time either fighting or having sex, according to D.B.

"It was their dynamic, and their combination of extreme mutual jealousy and possessiveness, in the context of her persistent demand and apparent obsession over a protracted period of time that he kill (Rengel), and his gullibility in the moment that resulted in the tragic death," she wrote.

The stabbing was "in response to an extreme, idosyncratic form of repetitive bullying in the service of sexual jealousy and control," Ramshaw said in her report.

A risk assessment Ramshaw conducted on D.B. found a 58 per cent likelihood he would violently reoffend within the next 10 years.

She also found he is of average intelligence.

However, while he was in Grade 12 at the time of his arrest he had only the equivalent of a Grade 9 education, with four credits. He also had a high rate of truancy and had been expelled several times.

Todorovic placed relentless pressure on D.B. through months of phone calls and text messages, instant messages and Facebook messages, urging him to kill Rengel by threatening to withhold sex because she believed the younger girl -- whom she had never met -- posed a threat to their romantic relationship.

"He was the immature, impulsive, aggressive male, whose loyalty, devotion, insecure attachment, perceived care-taking role and obsession with Ms. Todorovic were extreme, with sexual jealousy and fear of loss," Ramshaw wrote of D.B.

"She was the manipulative, unempathetic female who was also consumed by sexual jealousy, as well as a need for control, and who had an obsessive desire that he kill Stefanie."

The day of Rengel's death D.B. went to her house intending to fake that he had killed her, as he had once before in an attempt to quell Todorovic's nagging, the report reveals. She alternately threatened to leave him if he didn't kill Rengel or kill herself if he didn't murder the teen.

But he made up his mind to kill her and "didn't even think while I did it," he told Ramshaw.

"She screamed and that's what made me realize what I was really doing...at the second scream I started running away," he said. Looking back and seeing her still standing momentarily, D.B. thought she might live.

In a powerful victim impact statement made to the court Wednesday before the psychiatrist's report was presented, Rengel's mother Patricia Hung said she was haunted by not knowing her daughter's last moments.

"I lie awake at night wondering if he spoke to her before he plunged in the knife, over and over," Hung said.

"Did Stefanie beg him to stop, scream in pain, call my name? Did he just laugh at her and run? Did she know she was going to die?"

The opening day of the sentencing hearing included victim impact statements from Stefanie's mother and her younger brother. When they were finished, D.B. could be seen dabbing at his eyes with a tissue.

Ian Rengel, 13, said he feels lost without his beloved big sister.

"I never would have imagined it possible that a boy could be so messed up that he would take the life of an innocent person just because a girl told him to," he said.

"Every day there are things that I need her for. I'm older now, almost 14, just like her and I feel like she only knew me as a little kid. I wish she could see me grown up so that we could hang out more."

Ramshaw diagnosed D.B. with antisocial personality disorder. She said a mild conduct disorder complicated by attention deficit hyperactivity disorder and issues with aggression and impulsivity led her to the diagnosis.

Ramshaw found D.B. has "engaged in violent behaviour persistently throughout his life," beginning from a very young age, and including an "extraordinary number" of school suspensions due to fights. His triggers for anger are vast and D.B. was also in the 94th percentile for social maladjustment.

But D.B. is also able to express remorse and has a capacity for empathy, which is in contrast to psychiatric assessments of Todorovic.

The hearing continues Thursday.