Double amputee Oscar Pistorius, 27, once a national icon for reaching the pinnacle of sport, is accused of murdering Steenkamp, a law graduate and model, at his home in Pretoria on Valentine's Day last year.
The defence says Oscar Pistorius, nicknamed the 'Blade Runner' after his carbon-fibre prosthetic running legs, shot Reeva Steenkamp through a locked toilet door in self-defence, believing she was an intruder, and that therefore he should be acquitted.
State prosecutor Gerrie Nel has spent the trial, which began in March, portraying Oscar Pistorius as a gun-obsessed hothead who deliberately shot Steenkamp, 29, four times through the door of the toilet, where she was taking refuge after an argument.
Defence lawyer Barry Roux said during his closing arguments that psychological evidence had proven the track star had a heightened fight response because of his disability.
"You're standing at that door. You're vulnerable. You're anxious. You're trained as an athlete to react. Take all those factors into account," Roux said, adding that Pistorius had felt exposed because he was standing on the stumps of his legs.
"He stands with his finger on the trigger, ready to fire when ready. In some instances a person will fire reflexively," he added. "That is your primal instinct."
Roux also argued that prosecutors had only called witnesses who supported their argument and not other key people, including police officers, who he said would have undermined their case.
On Thursday Nel said Pistorius had told "a snowball of lies" and had called on Judge Thokozile Masipa to convict the track star of intentional murder, a crime which could land him with a life sentence.
A potential lesser charge of culpable homicide - comparable to manslaughter - could carry a sentence of about 15 years.
Pistorius also faces three separate charges, including two counts of discharging firearms in public and possession of illegal ammunition, all of which he denies.
To arrive at a verdict, Masipa and her two assistants will have to weigh up the credibility of testimony on both sides, including that of Pistorius, who endured more than a week of torrid cross-examination during which he broke down repeatedly.
In the absence of a jury, experts say the crux of the case is whether Masipa accepts or rejects his version of events.
Nel has called for Pistorius' evidence to be rejected from the judge's consideration because it was "devoid of any truth" and the athlete contradicted himself when he said during cross-examination that he fired both accidentally and deliberately.
Masipa, only the second black woman to be appointed a high court judge in post-apartheid South Africa, has to analyse more than 4,000 pages of evidence and it could take several weeks for her to come to a final verdict.
Roux said the trial should only ever have been on the charge of culpable homicide, rather than murder, because he said Pistorius had clearly shot Steenkamp by mistake.
Nel and Roux have focused much of their closing arguments on evidence from witnesses who say they heard a woman scream before a volley of shots, supporting the prosecution's position that the couple had an argument before Steenkamp was killed.
Roux went through the early morning of the shooting minute-by-minute during his wrapping-up, arguing that the witnesses were confused and contradictory about the sounds they heard.
He also spent time analysing photos he said proved the police had moved items in the couple's bedroom, countering a key claim by Nel that images of the room proved Pistorius' version of the events were impossible.
The courtroom duelling between Nel and Roux, both dynamic advocates with contrasting styles, has added to the drama in a trial that has captivated audiences around the world.
Nel, known as 'The Pitbull' because of his fierce cross-examination style and penchant for the dramatic, has been the perfect foil to Roux, an experienced criminal defence advocate whose meticulous eye for detail has put the squeeze on even the most composed prosecution witnesses.
The fathers of both Pistorius and Steenkamp were in court for the first time this week. The track star's aunt embraced Steenkamp's father before the trial resumed on Friday.
Steenkamp's dramatic death has shattered the image of Pistorius as an embodiment of triumph over adversity for both his Paralympic victories and his success against able-bodied athletes at the 2012 London Olympics.