Dr Merryll Vorster told the court events during his life, including the amputation of his lower legs as a baby and his late mother's habit of sleeping with a gun under her pillow contributed to his "increasing stress".
Pistorius' defence team said it would show that his feelings of "vulnerability" contributed to him shooting girlfriend Reeva Steenkamp by mistake last year, fearing she was an intruder hiding in a toilet cubicle at his home.
Prosecutors say Pistorius killed Ms Steenkamp intentionally after an argument. The Olympian is charged with premeditated murder.
Dr Vorster told the court she had interviewed Pistorius and some of his family members and friends.
She said Pistorius was more likely to respond to any threat with "fight" rather than "flight" and said he felt remorse over Ms Steenkamp's death.
Dr Vorster said: "He feels guilty and has developed a depressive disorder as a result."
The psychiatrist said the reactions of Pistorius in the early hours of 14 February 2013 would have been different to that of a "normal, able-bodied person without generalised anxiety disorder".
However, she said this would not have affected his ability to distinguish between right and wrong and it was up to the court to decide whether his anxiety disorder - a condition that he had suffered since childhood - had diminished his responsibility.
The defence is expected to conclude its case by the end of the week.