Oscar Pistorius, 27, was found guilty of the culpable homicide of Reeva Steenkamp last month but he was cleared of murder.
His psychologist was the first defense witness to speak at the sentencing hearing, which could last several days.
Oscar Pistorius faces up to 15 years in jail, although Judge Thokozile Masipa may suspend the sentence or impose a fine.
Thokozile Masipa said the South African athlete had acted “negligently” when he shot Reeva Steenkamp through a toilet door, but in the “belief that there was an intruder”.
Oscar Pistorius had strenuously denied murdering Reeva Steenkamp after a row on Valentine’s Day 2013, saying he shot her by mistake.
Reeva Steenkamp, a 29-year-old model and law graduate, was hit three times by bullets shot through a toilet door by Oscar Pistorius at his home in the capital, Pretoria.
Oscar Pistorius’ personal psychologist Lore Hartzenberg told the courtroom on Monday, October 13, that the Paralympic champion was “very emotional” during grief therapy sessions, which were often disrupted by his weeping and crying.
Lore Hartzenberg said the athlete felt “vilified and humiliated” by the media and social media networks, making him feel “utterly worthless”.
Judge hokozile Masipa is to hear legal arguments and testimony from a number of witnesses before passing sentence.
The judge said the state had failed to prove he intended to kill.
Oscar Pistorius, who has been on bail since the verdict, was also found guilty on a charge of negligently handling a firearm that went off in a restaurant.
He was acquitted of another charge of firing a gun in public, through the sunroof of a car, and of illegal possession of ammunition in the home where he killed Reeva Steenkamp.
The parents of Reeva Steenkamp said “justice was not served” after Oscar Pistorius was acquitted of murder.
June and Barry Steenkamp told NBC News of their “disbelief” that the court had believed Oscar Pistorius’ version of events.
Earlier, Arnold Pistorius, the athlete’s uncle, said the family was “deeply grateful” to the judge for finding him not guilty of murder and that a “big burden” had been lifted.
South Africa’s prosecuting authority said it was “disappointed” that Oscar Pistorius was not convicted of murder but said it would wait until after sentencing to decide whether to appeal.
Despite the conviction, the International Paralympic Committee has said Oscar Pistorius would be allowed to compete in future events.
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