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(CNN) -- Oscar Pistorius said he never picked on Reeva Steenkamp, and he didn't get a chance to tell her that he loved her before she died.
The athlete said he would like to meet Steenkamp's parents, but understood that they wouldn't want to.
"I don't think they will ever want to meet me," he said. " I am terribly sorry that I took the life of their daughter."
Prosecutor Gerrie Nel picked apart message exchanges between the couple, accusing the athlete of acting selfishly toward Steenkamp.
Messages between the couple revealed that he picked on her, screamed at her and humiliated her unfairly, he said.
Pistorius denied it.
'I didn't treat her badly," he said.
Asked if Steenkamp had lied he picked on her "incessantly," he replied: "She never lied."
Nel bluntly highlighted an incident where Steenkamp complained in a message that Pistorius asked her to stop chewing gum. He also read a message where the athlete accused her of flirting at a party and ignoring him.
"You were strong enough in that relationship to say stop your voices, stop your accents, stop chewing gum," the prosecutor said. But Pistorius said he gently told her to stop chewing gum before they got on camera.
"I was trying to help her but she obviously took offense," he said.
Pistorius said he never messaged her to tell her that he loved her despite her declaration in a message.
"We did a search ... the phrase 'I love you' appears twice on her phone, to her mother," the prosecutor said. "Apart from 'boo boo, baa, baa' you never wrote a long message to Reeva."
The prosecutor said Steenkamp wrote to her athlete boyfriend that "I'm the girl who fell in love with you," but he never responded.
"Because it was all about Mr. Pistorius," Nel said.
The Olympic sprinter was back on the stand Thursday for further cross-examination in his murder trial after a day of relentless and combative questioning. He has pleaded not guilty to murder and said he mistakenly shot Steenkamp at his house on Valentine's Day last year, thinking she was an intruder
The athlete maintained he would like to meet Steenkamp's parents to apologize.
"I don't think they will ever want to meet me," he said. "I am terribly sorry that I took the life of their daughter."
Pistorius said he did not discharge a firearm in a Johannesburg restaurant in an incident last year.
"The firearm discharged. I was trying to make it safe," he said.
Pistorius said he repeatedly took the blame for the incident, but the prosecutor tried to poke holes in his declaration.
"This is incredible, You never touched the trigger, the gun went off. You took the blame, your took responsibility but not one remembers," Nel said.
'You shot and killed her. Say it.'
A day before, a defiant Nel barked at the Olympic star on the stand.
"You shot and killed her. Say it -- 'I shot and killed Reeva Steenkamp,'" Nel told Pistorius on the first day of cross-examination Wednesday.
No one disputes that Pistorius killed Steenkamp. But the prosecution is trying to prove beyond a reasonable doubt that he did so knowingly and intentionally.
The 27-year old has admitted to the killing, but said he mistook Steenkamp for an intruder in the bathroom when he fired through the door and killed her.
Before Nel went after Pistorius, defense lawyer Barry Roux had tossed his client a question to drive that argument home. He asked Pistorius if he intentionally killed Steenkamp.
"I did not intend to kill Reeva or anybody else for that matter," he replied.
Later in the proceedings, as Nel probed him further, Pistorius insisted he thought he would be attacked when he heard noise coming from his bathroom that night.
"I had a fear, I didn't have time to think, I discharged my firearm ... I didn't intend to shoot at anyone, I shot out of fear."
Nel took the defense team by surprise Wednesday when he asked the athlete about a shooting range video in which Pistorius is seen firing at a watermelon, and then calling the impact "a zombie-stopper."
This prompted the defense to complain that the prosecution was staging an "ambush" by introducing evidence. The court was briefly adjourned as Judge Thokozile Masipa considered both positions. The defense later said it would not object to the video being shown in court.
"It makes me very upset to hear myself saying something like that," Pistorius said as he admitted to making the comment. But he insisted he was referring "to a zombie, not a human being."
Nel showed the court a graphic photo of Steenkamp's wounded head. Speaking of the watermelon in the video, the prosecutor said: "It exploded. You know the same happened to Reeva?"
Pistorius snapped, sobbing.
"I was there, I don't have to look at a picture," he said.
'My life is on the line'
Nel is going over the version of events that Pistorius provided in an earlier bail
application affidavit, and the one he's given on the witness stand this week.
Pistorius has told the court what he remembered from the night Steenkamp died, starting with the moment he opened his bathroom door after shooting through it and saw her bloodied body.
On Tuesday, he described tearfully how, gripped by fear, he shot Steenkamp dead through the locked toilet door, thinking she was an intruder.
The prosecution alleges Pistorius killed his girlfriend after they argued. Several witnesses have testified to hearing a man's shouts coming from the house, although they have also spoken of the terrified screams of a woman leading up to and during a volley of shots.
The trial has gripped South Africa, where Pistorius is considered a symbol of triumph over physical adversity.
His disabled lower legs were amputated as a baby, but he went on to achieve global fame as the "blade runner," winning numerous Paralympic gold medals on the steel blades fitted to his prostheses.
Only those in the courtroom can see Pistorius because he has chosen not to testify on camera. His testimony can be heard on an audio feed.
Steenkamp's mother, June, sat in court throughout Pistorius' three days on the stand.
In an interview with the UK's Mirror newspaper, she said she is still trying to decide whether the sprinter is acting as he defends himself against the murder charge.
"I look at Oscar the whole time, to see how he is coping, how he is behaving. I'm obsessed with looking at him, it's just instinctive, I can't explain it," she said.
"I keep thinking, 'let me see how he's taking this'. He has been very dramatic, the vomiting and crying."
Talking about Pistorius' apology this week, she said: "It left me unmoved. I knew it was coming. My lawyers had prepared me for it."
"I cried for the first time, 'Yes', but not because he apologized, because of the suffering and agony that my darling daughter went through and because I will never have her again."
The trial is scheduled to continue until the middle of May.
Judge Masipa will decide the verdict in collaboration with two experts called assessors. South Africa does not have jury trials.
CNN's Richard Allen Greene, Brent Swails, Emily Smith and Ben Brumfield contributed to this report
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