Gerrie Nel, who has become known as 'The Pitbull' for his hectoring style of questioning during his cross-examination of the Olympic and Paralympic runner, told a court in Pretoria that he was purely motivated by shooting and killing her at his home on February 14 last year.
At the end of five days of often dramatic courtroom confrontation, Mr Nel provided the stark summary of the prosecution's case against Mr Pistorious.
He said: "You fired four shots through the door whilst knowing that she was standing behind the door. She was locked into the bathroom and you armed yourself with the sole purpose of shooting and killing her."
Mr Pistorious, 27, who faces a lifetime in prison if convicted of the model's murder, but who insists the killing was accidental after he mistook her for an intruder in the toilet, replied: "That is not true."
The sportsman claimed he had pulled the trigger without thinking after hearing a noise behind the door, out of terror and fear that his and 29-year-old Ms Steenkamp's lives were in danger.
"I was extremely fearful, overcome with a sense of terror and vulnerability," said double amputee Mr Pistorius. "I didn't think about pulling the trigger, as soon as I heard the noise, before I could think about it, I pulled the trigger." The athlete's voice quivered as he recounted how he was "overcome with terror and despair" on finding her bloodied body slumped against the toilet after he broke down the door with a cricket bat.
"I was broken, I was overcome, filled with sadness," he told judge Thokozile Masipa, adding he urged Ms Steenkamp to hold on while he sought help from neighbours at his high security Pretoria residence.
Ms Pistorius gave a sometimes muddled account of the shooting, saying he feared for his life but also did not intentionally shoot at anyone. Mr Nel closed his cross-examination by inviting Pistorius to take the blame for shooting Ms Steenkamp, but the runner steered away from a direct response, saying only that he opened fire because he believed his life was under threat. That remark drew barbed follow-up questions from the prosecutor.
"We should blame somebody ... Should we blame Reeva?" asked Mr Nel, who has harshly criticised Mr Pistorius as someone who is unwilling to take responsibility.
"No, my lady," Mr Pistorius replied. Mr Nel asked him: "She never told you she was going to the toilet," Mr Nel said. Then he asked: "Should we blame the government?"
When Mr Pistorius responded with another reference to a perceived attacker in his toilet, Mr Nel asked: "Who should we blame for the Black Talon rounds that ripped through her body?"
He abandoned his line of questioning soon after the judge questioned whether he was asking the same thing in a different way.
Mr Pistorius insists he and Ms Steenkamp were in a loving, if fledgling, relationship, despite phone text messages read in court which pointed to some arguments. He read a Valentine's Day card his girlfriend got for him before her death."Roses are red, violets are blue," the card begins. "I think today is a good day to tell you that I love you," the message concludes, the last part in Ms Steenkamp's own words.
Mr Pistorius's prowess on the track using prosthetic limbs earned him fame, fortune and a clutch of Paralympic medals as the self-styled "Blade Runner". After the cross-examination, Mr Pistorius rubbed his eyes and briefly sank his head into the shoulder of a man who comforted him.
He took a tissue from his sister Aimee, who squeezed his arm reassuringly. Shortly afterwards, he listened attentively as Barry Roux, his chief lawyer, spoke to him in a low voice.
Mr Pistorius has broken down in tears on many occasions during the questioning, and at one point retched into a bucket on the witness stand after being shown grisly pictures of Reeva Steenkamp after the shooting on Valentine's Day last year. The defence later moved on to questioning its third witness, with the trial looking likely to run into next month.