During a third day of intense cross-examination, prosecutor Gerrie Nel accused the 27-year-old of having a selective memory on incriminating details of the night he killed Steenkamp in the bathroom.
The prosecutor nicknamed "the bulldog" also ripped apart the Paralympian's claim that he had rushed to the bathroom with a gun "out of instinct" that danger was lurking behind the door.
"I find your instinct strange," said Nel. "Instinct would have made sure Reeva was safe. That was never your instinct: to make sure she was safe."
Pistorius has been charged with murdering his 29-year-old model girlfriend and faces a life sentence if convicted. He has said he fired the shots accidentally and did not mean to kill anyone.
But Nel has been seeking to pick apart Pistorius's version of events.
As Nel unleashed volley after volley of questions on the details of what happened that Valentine's Day night and over apparent discrepancies in Pistorius's accounts, the sprinter grew increasingly agitated.
"I'm not looking for an excuse, if I don't remember it I don't remember it!" Pistorius said, breaking down during one exchange about the placement of objects in his bedroom.
"This is the night I lost the person I most cared about, I don't know how people don't understand that," he said through sobs.
'Why are you changing your evidence?'
But Nel did not let up, accusing the double amputee sprinter of changing his version of events and of deliberately not remembering details that could be detrimental to the athlete's case.
"Why are you changing your evidence?" Nel asked.
"I'm tired my lady," Pistorius said.
"I'm not convinced about your answer now, I think you're trying to cover up for lies," Nel said.
The athlete said he fired through his locked bathroom door thinking the model and aspiring actress was an intruder, as he had a heightened fear of being a victim of crime.
But Nel, who has charged the Paralympian with Steenkamp's premeditated murder, dismissed the claim.
"You're standing there, you're facing the door. Why would you think somebody was attacking you?"
"In fact, you knew Reeva was behind the door and you shot at her!" he said.
"It's not true, milady," Pistorius replied softly, before the court session ended for the day.
Before starting a third day of pummelling cross-examination, Nel said Steenkamp's mother confirmed that Pistorius had requested a meeting with the family.
"But they weren't ready" to meet the Paralympian, Nel told the court, adding that Reeva's mother June Steenkamp wanted that on the record.
The prosecutor had earlier attacked Pistorius for making a public apology to the Steenkamps when he took the stand on Monday, saying the double amputee sprinter was playing to the gallery.
But Pistorius said then that he had once asked to meet his girlfriend's family in person to apologise.
June Steenkamp was quoted in a newspaper this week saying the athlete had gone from "hero to devil" after killing her daughter.
"My presence unnerves him, I'm sure of it. He's answerable to me," Steenkamp told Britain's Daily Mirror.
Friday marked the end of a difficult week on the stand for Pistorius, during which he has been accused of lying and fabricating evidence.
"Your version is so improbable that nobody would ever think it's reasonably, possibly true it's so impossible," Nel thundered during the cross-examination on Thursday.
"Your version... is a lie," he charged.
Confronted with crime scene pictures that showed his version to be improbable, Pistorius said police tampered with objects when they came to his upmarket house.
Someone moved fans, pulled the duvet onto the floor and opened the curtains, he said.
"Is this one big conspiracy?" asked Nel with incredulity. "They would do all this to you?"
The double-amputee faces a life sentence if convicted.
Pistorius's cross-examination is a stern test of both his version of events and of his resolve.
Defence lawyer Barry Roux is expected to call up to 17 witnesses in the remainder of the case.
Originally scheduled to run for three weeks, the case has been extended until mid-May but could go on longer.