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Prosecutor Gerrie Nel told the athlete to look at a graphic image of his girlfriend's bleeding head wounds and take responsibility for inflicting them.
Pistorius lost his composure and cried when shown the close-up photograph of Steenkamp after Nel told him her head had exploded when he shot her repeatedly through a locked toilet door last year.
"You know the same happened to Reeva's head, it exploded," said Nel, as Pistorius's family flinched in the public benches.
"It had the exact same effect, the bullet that went into her head."
Nel was referring to Pistorius target practising on watermelons with heavy calibre ammunition in footage that was shown in court in a surprise move by the State.
Pistorius tearfully protested: "I remember... my fingers touched her head, I do not have to look at a picture."
Pistorius's lawyer Barry Roux leapt up to object to the prosecutor's questioning and Judge Thokozile Masipa adjourned court to allow the Paralympian time to calm down, as she has done several times before this week,
It was Pistorius's third day on the stand after two in which, under questioning from Roux, the double amputee athlete spoke first of his early life, his rise to fame and his religious faith before giving a detailed account of the shooting in the early hours of Valentine's Day last year.
Pistorius told the North Gauteng High Court in Pretoria he believed he was protecting himself and Steenkamp from an intruder and collapsed wailing when he recalled forcing open the bullet-riddled door to find his dying girlfriend.
Asked by Roux on Wednesday, whether he intended to cause her death, Pistorius firmly responded: "I did not intend to kill Reeva, My Lady or anybody else for that matter."
With that Roux's questioning ended and Nel launched into cross-examination, seeking to prove the State's contention that Pistorius shot Steenkamp with intent after the couple had an argument.
Nel began with the statement: "Mr Pistorius you were and still are one of the most recognised faces in the world. You are a model for disabled and able-bodied sportsmen all over the world."
Pistorius replied softly that this was the case before he made "a mistake".
Nel snapped: "You made a mistake? You killed a person. You shot and killed Reeva.
"Won't you take a responsibility for that?"
The prosecutor then reminded Pistorius that he had testified that he was a practising Christian and asked whether he always told the truth.
"I try not to lie," came the reply.
Nel went on to ask Pistorius what a "zombie stopper" was and if he had been in the presence of someone using the term.
When Pistorius said no, Nel introduced the Youtube video amid protest from the defence team.
In it Pistorius was heard commenting that the watermelon he blew apart was "softer than brains" and comparing the bullets he fired to "zombie stoppers".
Nel also pointed to discrepancies between Pistorius's statement at his bail hearing last year and his testimony this week in his murder trial, and asked repeatedly whether he had tailored his evidence as he listened to the case unfold in court.
In sometimes rambling answers, Pistorius said this was not so.
"If you argue or think about other evidence, you will get into trouble," Nel warned.
Cross-examination was set to continue after lunch.
Pistorius in 2012 became the first double amputee to compete in the Olympics.
If found guilty of murder, he risks life in jail.