reeva steenkamp's cell phone exposing the inner workings of what the prosecution portrayed as I troubled relationship. I'm scared of you sometimes and how you snap at me and how you will act to me.... See More
reeva steenkamp's cell phone exposing the inner workings of what the prosecution portrayed as I troubled relationship. I'm scared of you sometimes and how you snap at me and how you will act to me. Reporter: Intimate text messages the couple shared in the weeks before Oscar Pistorius shot and killed her in his bathroom last Valentine's day. I get snapped at and told my accent and voices are annoying. I touch you to show you I care. You told me to stop. Stop chewing gum. Do this, don't do that. Reporter: Pistorius claims he mistook her for an intruder. But the prosecutor building a case that this was not mistaken identity but murder. Possibly triggered by a violent argument. The key clues to which may reside in steenkamp's cell phone. The expert said most of their hundreds of messages were Normal exchanges between a couple but in some read in court Pistorius sounded jealous and quick to anger. I was upset that you left -- that you just left me after we got food to go talk to a guy, and I was standing tight behind you watching you touch his arm and ignore me. Reporter: Her feelings hurt, writing back -- I regard myself as a lady, and I didn't feel like one tonight. After the way you treated me. I can't be attacked for dating you. The one person I deserve protection from. Reporter: Reeva's friends portrayed her as sunny and supportive. She was just always happy and smiling. And always had the most amazing outlook on life. Reporter: But they also told ABC news that reeva had been in a bad relationship in the past. She knew the signs. She'd been in an abusive relationship. Reporter: The greatest irony. She was going to give a talk to schoolgirls at a high school that day, actually, about abuse. Reporter: But Pistorius, the seemingly invincible athlete, was also vulnerable. Especially in his youth, when he wrote he was bullied at school. His mother would die tragically shortly after that. Then Oscar's greatest triumphs came in a tumble. Olympic victory in Athens, beijing, and London. In his adulthood Pistorius would take steps to protect himself from threats that went beyond bullying, surrounding himself with new friends and new weapons. I heard he had guns. Reporter: Fame and success that seemed to change him. What would you do if you had heard that your client had been driving in a car at 260 kilometers an hour, 180 miles an hour? Those are things that really shocked me when I heard. How much of that do you think is media hype and how much of it do you think is Oscar Pistorius changing? You know, I think it's Oscar Pistorius changing. I mean, they were going 260. He was shooting the gun out the sunroof. I mean, he did do those things. Reporter: Ebba gudni never saw that jealous side of Pistorius. He's got a big warm heart. Reporter: Her son like Pistorius was also born without fibulas. They met in her native Iceland. He takes care of people. He's funny. He's positive. He's very brave. And he always tries to make sure that everybody's okay. Reporter: And early today the most harrowing testimony yet. The thing that I remember the most, and I don't think I'll ever forget, was the screaming. Reporter: In a trial so much about sound, we could only hear the testimony of Oscar Pistorius's neighbor, Annette step. I could still hear the screaming up until the first or the second set of shots. Reporter: She said she heard a woman's screams after an estimated ten minutes of commotion. She then heard shots and then silence. There were no eyewitnesses that night. Only so-called earwitnesses. The fragments of information as fleeting as those text messages. I'm scared of you sometimes. Reporter: But today these fragments according to the prosecution painting a very different tale than the one told by Oscar Pistorius of a peaceful evening at home and a case of mistaken identity. In South Africa the prosecution needs only to convince one person. There is no jury, just the judge. And only one man who knows the truth. And Oscar Pistorius could testify as early as this week. For "Nightline" I'm Matt Gutman in Pretoria, South Africa.
This transcript has been automatically generated and may not be 100% accurate.