The 27-year-old Paralympian wrapped up three days of emotional and harrowing defence testimony by denying he wanted to kill girlfriend Reeva Steenkamp or the intruder he believed to be behind a toilet door in his home on the night of Valentine's Day last year.
"I did not intend to kill Reeva, milady, or anyone else," he told the murder trial, in his strongest rebuttal yet of charges against him.
Pistorius claims he mistook Steenkamp for an intruder.
He faces a life sentence if convicted of her murder.
But he could also face a murder charge if the prosecution shows he intended to kill when his life was not threatened -- regardless of whether he knew who was behind the door.
After Pistorius's denial, prosecution lawyer Gerrie Nel began a searing cross-examination, immediately confronting Pistorius with the magnitude of his actions on February 14, 2013.
After Pistorius conceded he made a terrible "mistake", Nel -- nicknamed "the bulldog" for his tenacious courtroom performance -- was incredulous.
"You made a mistake? You killed a person, that's what you did!" he thundered.
"You shot and killed her, won't you take responsibility for that?"
Pistorius's cross-examination is expected to be a key point in his trial, a stern test of both his version of events and of his resolve.
During five weeks of proceedings the world-famous athlete has appeared fragile, frequently crying in court and becoming physically sick when the gruesome details of Steenkamp's injuries were discussed.
He is likely to remain on the stand for the remainder of this week as his extensive testimony probed and picked at by Nel, who once successfully prosecuted a South African police commissioner.
'Nothing more I could do'
In calling Pistorius as a witness in his own defence, his handpicked legal team tried to counter the state's portrayal of him as reckless and obsessed with fast cars and guns.
Defence lawyer Barry Roux gently painted the portrait of a young man who likes dogs and carries out charitable acts, but who was marked by a fatherless childhood, the early death of his mother, physical disability and recurring crime.
Pistorius had described how he met Steenkamp, a vivacious law graduate, and how they quickly grew closer and planned their future together.
He demonstrated remorse by apologising publically to Steenkamp's family for what happened, a display that could be presented as a mitigating factor in any sentencing.
Pistorius emotionally recounted how he "was overcome with fear," when he believed an intruder was in his bathroom.
"Before I knew it, I'd fired four shots at the door."
He later gave horrifying testimony about his vain attempts to stem the 29-year-old model's blood loss and save her life using plastic bags and utility tape.
"I was really trying to stop the bleeding, I was really trying to help Reeva breathe," said Pistorius, still struggling to retain his composure.
"I was trying to hold Reeva's head with my hand to put pressure on it," he said, outlining the brutal and ultimately fatal results of his gunshots to her hip, arm, hand and head.
"Reeva had already died while I was holding her," he said. "There was nothing more I could do for her."
Pistorius's testimony also touched on three other unrelated charges: firing a gun through a moving car's sunroof, then again in a crowded restaurant, and also the possession of illegal ammunition.
He has pleaded not guilty to all the charges against him.
Defence lawyer Barry Roux is expected to call up to 15 more witnesses in the remainder of the case, to testify on ballistics, urine emptying, damage to the toilet door, sound, and "disability and vulnerability".
Eventually set down for three weeks, the trial could run until mid-May, possibly even longer.