Good evening and thanks for joining us. Paralympian Oscar Pistorius has been in court for O a month without speaking publicly. Until now. The athlete once famously known as the blade runner finally... See More
Good evening and thanks for joining us. Paralympian Oscar Pistorius has been in court for O a month without speaking publicly. Until now. The athlete once famously known as the blade runner finally taking the stand today in his murder trial. He admits he shot and killed his girlfriend, reeva steenkamp. But how did it happen? And why is he apologizing now? ABC's Matt Gutman is in south Africa with this report from inside the courtroom. Reporter: It was a moment anticipated for 14 months. Oscar Pistorius solemnly making his way to the witness stand, breaking his silence with a sob-filled apology to the woman he loved. The woman he's charged with murdering. There hasn't been a moment since this tragedy happened that I haven't thought about your family. I was simply trying to protect reeva. Reporter: The blade runner once indomitable on the track. Now his voice quivering, barely audible. I can promise you that when she went to bed that night she felt loved. Reporter: After Pistorius made his way to the witness stand he walked right here and addressed reeva steenkamp's family. The defense would not allow him to be videotaped so it was audio only. But here in court we could see his lip quivering as he tried to talk to them. We could tell he hadn't slept in days, his eyes sunken and dark. And for the rest of his nearly three-hour testimony he sat here like this, hunched over. Steenkamp's mother sat stone-faced, seemingly unmoved. Unlike others in the courtroom, who swept. Legal analysts say his testimony is the hinge on which this trial to could turn. Pistorius admitted to shooting reeva through the closed door of his bathroom at home on Valentine's day last year. Pleading not guilty, he claimed he mistook the model and reality star for an intruder. We're only interested in with in H whether in his own mind he thought he was acting unlaflly in that moment. Reporter: And what would unfold for the next three hours in a stuffy Pretoria courtroom would essentially be the blade runner's autobiography, perhaps an attempt to humanize him in the eyes of the world. And the judge who will decide his fate. Starting with his traumatic last year. Do you have difficulty in sleeping? I'm scared to sleep. I have terrible nightmares. About things that happened that night. I wake up and I can smell the blood. And I wake up to being terrified. His defense attorney asking him to describe his descent into paranoia. I've got a security guard that stands outside of my front door at night. Reporter: It is a trauma so thorough, said the broken man testifying, that after living through the ghastly tableau shown in crime scene photos in court, he's on medication for depression and throughout still professing his love for reeva. I was kind of taken aback by her and just bowled over by how much I fell for her. Reporter: He says he always kept his gun close, under his bed, he says to protect his beloved girlfriend and himself. Pistorius grew one a healthy fear of the outside world. His mother divorced from his father when he was just 6 years old, kept a pistol beneath her pillow. Her fear of South Africa's rampant crime would become his. Living with my mother we were broken into on several occasions. My father has been hijacked twice. My brother was in an intense hijacking. Have you ever been a victim to crime? Yes, I have. Reporter: And that sense of insecurity, that fear of crime is shared by nearly all south africans. After all, the incidence of burglary, home invasion, rape, murder, so much higher here than in the U.S. And the paralympic champ said he felt it more keenly than others, especially when he wasn't wearing his prosthetic legs. It just throws my weight off completely. My dog could knock me over without my process tet UK legs on. Reporter: Pistorius appears to be a shell of the fearless olympian he was a year ago. His former agent was there for the good times. How high was he flying before the shooting? He was an icon. The world loved him. He was unbelievable. Reporter: He couldn't believe the transformation when he saw Pistorius in the weeks before the trial. I didn't recognize him. And he came, he said can I have a hug. And he gave me a hug. He was shaking. And he said, not too many people love me anymore. Reporter: It is this portrait of a man crippled by guilt and depression who'd always felt vulnerable, who felt the need to arm himself against the outside world, which could actually be the key to his defense. He'll of course raise the fact that he is disabled, that he has a heightened sense of vulnerability compared to other people in the same position. Reporter: His story winding down, Pistorius seemed to sink in his chair. Did you sleep last night? No, sir. Reporter: His attorney called for an adjournment for the day, and the judge assented. Well, he does look exhausted. He does sound exhausted. Reporter: Walking out of court, the once invincible olympian looked shattered. But he returns to this courtroom tomorrow with the final chapter of his tale left to tell, a Valentine's day tragedy that ends with his love dead in his arms, a murder charge against him, and a single question. Why did he pull that trigger? I'm Matt Gutman for "Nightline" in Pretoria, South Africa.
This transcript has been automatically generated and may not be 100% accurate.