Oscar Pistorius admitted that he had no reason to fire the four shots that killed Reeva Steenkamp as the prosecutor Gerrie Nel accused him of lying to the court.
Nel insisted that the Olympic athlete has repeatedly failed to take responsibility for his actions and quizzed him about his three other gun-related charges as the cross examination of Pistorius continue.
Pistorius failed to explain why a gun in his possession went off in a busy restaurant last year while the court heard him portrayed as a bully in his relationship with Ms Steenkamp.
The double-leg amputee denies the premeditated murder of Ms Steenhamp at his home on Valentine's Day last year, Under intense scrutiny from Nel, Pistorius said he was sure an intruder had entered his house but said he did not intend to shoot anyone.
I didn't have time to think about if I wanted to or didn't want to," he told the the high court in Pretoria.
Nel challenged: "We know for a fact there were no intruders in your house that night, we know for a fact there was no ladder against the wall. We know for a fact that you had no reason to shoot, objectively speaking." Pistorius told the judge: "That's correct, my lady."
Pistorius explained: "I heard a noise coming from the toilet that I interpreted as someone coming out to attack me."
Mr Nel pressed him: "Accidentally your fingers pulled the trigger?"
When Pistorius agreed, Nel said: "At the intruder?"
Pistorius countered: "At the door."
At this point Pistorius started to become emotional claiming it was an accident. The court was temporarily adjourned in order to give PIstorius time to compose as the pressure of being cross examined for four days took its toll.
Nel then pressed Pistorius' claim that he had told Ms Steenkamp to phone the police upon hearing a noise in the toilet.
He said it was the "most improbable" part of the account.
"At that stage when you shouted at Reeva to phone the police, she was 3m away from you in the toilet, but she didn't say anything," he said.
"Why would she not say anything?"
Mr Pistorius could not respond to the question.
"She wasn't scared of an intruder," Nel said. "She was scared of you."
He went on: "Are you sure that Reeva did not scream after the first shot?"
Pistorius broke down again before answering: "At no point did Reeva shout out or scream."
The Paralympic champion said he had heard the window open and close before grabbing his gun.
"On the morning of the 14th I was sure of what I heard," he said.
"My whole being was fixated on this person that I thought was in the bathroom."
The court has now been adjourned until Monday.
Pistorius faces life imprisonment if found guilty of the murder of Ms Steenkamp. If he is acquitted he will be charged with another form of culpable homicide, which carries a maximum sentence of £15m.