Pistorius will learn his fate when the South African court reconvenes July 6 for his sentencing.
Defense attorney Barry Roux asked the Paralympic gold medalist to remove his prosthetic legs and stand in front of the court Wednesday while the lawyer described the events of the night of Valentine's Day 2013.
Pistorius shot Steenkamp dead through a bathroom door in their home. He has said he thought an intruder was in the bathroom.
Looking frail, Pistorius, who had changed out of his suit and into shorts and a T-shirt, required assistance as he walked unsteadily across the courtroom Wednesday. He stared at the floor and started weeping as Roux spoke about the killing.
Murder carries a minimum sentence of 15 years in South Africa, but the defense is trying to secure a lesser sentence
by arguing there are
mitigating factors, including Pistorius' physical and emotional disabilities.
"He suffers from an anxiety disorder. We know that uncontested evidence that when he was on his stumps, his balance was seriously compromised, and without anything he won't be able to defend himself," Roux said.
Pistorius was not wearing his prostheses at the time of the shooting, and Roux painted a picture of a fearful man on his stumps, trying to protect a loved one.
"He was anxious, he was frightened. ... He was suffering from anxiety disorder, and that's not gone," Roux said. "This must all be seen in context of his disability."
The lawyer added it was "understandable that a person with disabilities such as that of the accused would certainly feel vulnerable when faced with danger." He said that Pistorius' disability also means he would face a harder time in jail.
Defense: Pistorius will punish himself more than court can
Roux told the court that the "real facts" of the case had become clouded by false perceptions about the killing and Pistorius.
"Some people refuse to sit back and look at the objective facts," he said, asserting the killing had "nothing to do with gender violence."
He insisted that Pistorius had believed Steenkamp was in bed and that an intruder was behind the bathroom door. He said Pistorius had not intended to kill the target and that he had attempted to save Steenkamp's life.
Pistorius, a first-time offender, has paid financially and emotionally for his actions, and will punish himself all his life for what he did, Roux added.
"We say the accused has lost all of his essence and his career is gone," he said.
"The accused can never resume his career. The accused has punished himself and will punish himself for the rest of his life far more than any court of law can punish him."
Prosecutor: Steenkamp's father the 'broken man'
Chief prosecutor Gerrie Nel followed Roux's closing arguments by stating that Steenkamp's father, Barry, who gave emotional testimony
Tuesday, was the "broken man," not Pistorius.
He said the defense has been focused on the consequences on the killer, rather than the victim's bereaved family.
"The victims in a matter like this must be the mother, the father and the family of the deceased. The sentence needs to be victim-centered," he said.
In his tearful appearance on the stand Tuesday, Barry Steenkamp said he had been "devastated" by his daughter's murder and "changed completely."
"I don't wish that on any human being, finding out what happened," Steenkamp said. "I ended up having a stroke and so many things since then have happened to me."
Nel pointed to Pistorius' accomplishments as an athlete as evidence of his ability to overcome limitations.
Nel told the court the Paralympian had not taken medication he had been prescribed for depression and anxiety at the time of the attack.
"Maybe he didn't want to get better," he said. "The tablets were still in the packaging."
He also said Pistorius had been a "poor witness," who had put forward differing accounts about the night of the murder.
Nel said the defendant could not truly be remorseful when he had failed to give a satisfactory account of why he had fired the fatal shots, and he called for a sentence of at least 15 years.
Judge Thokozile Masipa granted an application by Nel, with the support of the Steenkamp family, to release photographs of the victim's injuries.
Cousin: Reeva didn't love him
Earlier Wednesday, Pistorius had been overcome with emotion as Reeva Steenkamp's cousin, Kim Martin, told the court that the victim didn't love him.
"I know she (Reeva) liked Oscar, I don't think she loved him. She was fond of him. She had a soft spot for him," Martin said Wednesday.
She testified that the killing three years ago irrevocably changed her family.
"As a family we will never be able to carry (on) normally," she said.
"I saw my dad, for instance, cry for the very first time when Reeva died, and then for the second time when he heard I would have to testify again.
"As far as I'm concerned (Reeva's father, Barry, is) a broken man ... the guilt of the father not being able to protect his daughter, it's very difficult for him."
She accused Pistorius of never apologizing for shooting Steenkamp. "All we've ever wanted was the truth. Oscar's version changed so many times. ... I have never heard him apologize for shooting and killing Reeva behind that door."
New sentencing ordered
Pistorius has faced a new sentencing hearing after South Africa's Supreme Court of Appeal ruled in December that his fatal shooting of Steenkamp was murder, not culpable homicide.
On Monday, the first day of the hearing, clinical psychologist Jonathan Scholtz told the court that Pistorius is a "broken man," not capable of testifying.
He said Pistorius was still on medication for post-traumatic stress disorder and depression, and that he would recommend hospitalization if Pistorius were a regular patient.
The hearing may be the final chapter of the widely watched trials that marked a fall from grace for the athlete, nicknamed "Blade Runner,"
the first double-amputee
to compete in the Olympics
After a nearly 50-day trial stretched over seven months, Pistorius was convicted of culpable homicide (much like manslaughter) in September 2014.
Masipa, the judge, ruled the sprinter had acted negligently when he shot Steenkamp four times through a locked bathroom door but that he didn't do it intentionally. The Supreme Court of Appeal later changed that conviction to murder.