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 defence, testified that a series of events in Pistorius' 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 defence-led testimony that the double amputee's vulnerability and disability was at the centre of his case of a mistaken killing. But prosecutor Nel questioned Monday if Pistorius was merely trying one of a number of defences for shooting his girlfriend.
"There must be some reason why Mr. Roux decided to call this witness. It might be the third defence that we have," Nel said.
Pistorius, the first amputee to run at the Olympics, testified at his trial that he fired his gun accidentally at the toilet door. That appeared to contradict his initial statement in court documents last year that he shot in self-defence because he believed his life was in danger.
Nel said if Pistorius had genuine anxiety issues, Judge Thokozile Masipa should order him to be evaluated. Responding to a question from a reporter after the trial was adjourned, Pistorius said "Go read the law."
As the athlete sat in the Pretoria courtroom earlier Monday making notes and occasionally looking up at the witness stand, Vorster outlined his apparently unhappy childhood and a life story in contrast to the smiling, triumphant disabled runner who made history at the London Olympics in 2012.
Vorster's testimony raised the question of whether Pistorius was now claiming "diminished responsibility" for the shooting, Nel said, because of a possible mental illness. Nel also asked the psychiatrist if someone who was suffering from such an anxiety disorder, and had access to guns, would be a danger to society. Vorster said the person would, indeed, be a danger.
Vorster's testimony came at the start of the eighth week of the globally televised trial, and a day before Pistorius' defence had predicted it might wrap up its case. It now appears unlikely that the defence will rest Tuesday. The prosecution's cross-examination of the psychiatrist could continue on day 31 of proceedings after Nel asked for more time to look at her report on Pistorius.
Vorster reached her opinion after meeting with Pistorius on two occasions this month, and also from interviews with his family, friends and agent, she said.
Pistorius and his brother and sister "were reared to see their external environment as threatening," Vorster said, and this played a part in his actions on the night of the shooting.
Vorster said Pistorius was more likely to try and "fight" what he thought was an intruder than run away, because his disability meant it was harder for him to flee. Pistorius was on his stumps when he shot Steenkamp.
Imray reported from Stellenbosch, South Africa.
Also on HuffPost