Oscar Pistorius has a "profound fear of crime" and is vulnerable because of his disability, his murder trial has heard.
Giving evidence for a second day, sports medicine Professor Wayne Derman said the Paralympic athlete had developed an "exaggerated fight response" and suffered "significant stress and anxiety".
Derman, who treated Pistorius for six years, said this was responsible for the "horrific tragedy".
Describing how Pistorius' disability had affected his life, Derman said the athlete had lived with a "lifetime of real and learnt vulnerability".
The athlete's lower legs were amputated when he was a baby becuse he was born without fibula bones.
Derman is the final witness being called by the defence in the trial in Pretoria.
Pistorius denies deliberately killing his girlfriend Reeva Steenkamp on February 14 last year, claiming he fired four shots through a toilet door - three of which struck the 29-year-old model - because he thought there was an intruder.
Under cross-examination, the prosecution claimed Derman's evidence could not be objective because of his relationship with Pistorius - something he denied.
On Monday, the court heard the result of a psychiatric report, which ruled the 27-year-old athlete was not mentally ill when he shot Steenkamp dead.
He had spent a month being assessed at the Weskoppies hospital in Pretoria ahead of the ruling.
The trial has been adjourned until Monday (July 7).
July 2014: Pistorius a "suicide risk", says psychologist's report
July 2014: Neighbours unlikely to have heard woman's screams on night Pistorius shot girlfriend dead, court told
June 2014: Pistorius not mentally ill when he killed girlfriend, psychiatric report rules
June 2014: Pistorius' murder trial to resume after mental health tests completed
May 2014: Pistorius sells house where he shot dead girlfriend Steenkamp