The judge has not yet ruled on prosecutor Gerrie Nel's application that the court considers a period of mental evaluation for the world-famous runner.
Dr. Merryll Vorster, a psychiatrist called by the defense, testified that a series of events in Pistorius's turbulent life, including the amputation of his lower legs as a baby, his parents' divorce, his late mother's habit of sleeping with a gun under her pillow and his own fear of violent crime contributed to his "increasing stress."
"Overall, Mr. Pistorius appears to be a mistrustful and guarded person," Vorster testified. She said he has "many features of anxiety."
Vorster said Pistorius' anxiety combined with his physical disability may have caused him to act differently from other people when he shot four times through a toilet stall door in the early hours of Feb. 14, 2013, killing girlfriend Reeva Steenkamp.
Pistorius, 27, claims he mistook Steenkamp for a dangerous intruder when he shot her with his licensed 9 mm pistol in the pre-dawn hours. Prosecutors say he killed the 29-year-old model after an argument and shot in anger and not fear last Valentine's Day.
Pistorius' chief lawyer Barry Roux said at the start of defense-led testimony that the double amputee's vulnerability and disability was at the center of his case of a mistaken killing. But prosecutor Nel questioned Monday if Pistorius was merely trying one of a number of defenses for shooting his girlfriend.