In a dramatic opening to his cross-examination of Pistorius, prosecutor Gerrie Nel made him admit he had killed Steenkamp then later confronted him with the photograph showing the side and back of her skull, her hair matted with blood and brains.
"Have a look there. I know you don't want to because you don't want to take responsibility," Nel said to gasps from the packed public gallery.
"Take responsibility for what you have done," Nel persisted, his voice rising almost to a shout.
Pistorius responded by burying his head in his hands in the witness stand, rocking from side to side and weeping.
The double amputee sprinter, once revered across the world for his triumph over adversity, faces life in prison if convicted in the Pretoria High Court of the murder of Steenkamp, a 29-year-old law graduate and model.
His defence hinges on his contention that he thought he was firing at an intruder when he shot Steenkamp through a toilet door in his luxury Pretoria home on February 14 - Valentine's Day - last year.
During the 18-day trial, Nel, renowned as one of South Africa's toughest state attorneys with the nickname 'The Pitbull', has sought to show the 27-year-old as a gun-obsessed hot-head.
Early in his questioning, he asked Pistorius if he knew what a "zombie stopper" was, to which the defendant answered no.
After a brief adjournment, the court then viewed video footage broadcast before the trial by Britain's Sky News of Pistorius firing a .50 calibre handgun at a watermelon at a shooting range.
As the melon disintegrates, Pistorius says off-camera: "It's a lot softer than brains. But (bleep) it's like a zombie stopper."
Nel then pushed the track star, saying he had shot the melon because he wanted to see what a bullet hitting a person's head looked like.
"You know that the same happened to Reeva's head. It exploded. I'm going to show you," he said, before projecting the forensic photograph of Steenkamp's head on the court monitors.
Steenkamp was hit by three of four 9 mm rounds fired by Pistorius through the toilet door. One hit her behind the right ear, killing her almost instantly, pathologists had earler told the court.
Pistorius acknowledged responsibility but refused to lift his head.
"I don't have to look at a picture. I was there," he said.
Television stations carrying the feed from the court apologised to viewers as the graphic image was broadcast live.
With no direct witnesses, Nel's main task is to pick holes in Pistorius' testimony and cast doubt on his statements about a perceived burglar, a common fear in crime-obsessed South Africa.
Crucially, he forced Pistorius to concede that he did not go out on to the balcony in the middle of the hot and humid night to bring two fans inside - the instant during which, he said in a sworn affidavit submitted at his bail hearing, that he believed Steenkamp went to the toilet without him realising.
"You can't get away, Mr. Pistorius," Nel said.
Pistorius was forced to concede: "My memory isn't very good at the moment."
Earlier, he described his frantic attempts to revive Steenkamp after he found her lying barely alive on the toilet floor and how she had died minutes later in his arms, her blood pouring over his body.
"I checked to see if she was breathing and she wasn't," he said. "I could feel the blood was running down on me."
After several attempts, he managed to carry Steenkamp down stairs where neighbours tried to administer first aid before paramedics arrived.
But Pistorius said he knew that Steenkamp - with whom he said he was planning to buy a house - was already dead.
"Reeva, Reeva had already died whilst I was holding her, before the ambulance arrived, so I knew there was nothing they could do for her."
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