Prosecutor Gerrie Nel, known as the "bull terrier" for his aggressive style, discussed discrepancies in Pistorius' testimony and a shift in Pistorius' defense, claiming the legless athlete initially claimed that he fired at what he thought was an intruder and later said he didn't mean to fire but the gun went off because he was startled.
"We have two defenses, and we want the court to pick one," Nel said.
The prosecutor compared the trial to a relay race and said the legless athlete had "dropped the baton of truth."
Pistorius, 27, is charged in the Valentine’s Day 2013 shooting death of Steenkamp. Prosecutors argue that he intentionally shot Steenkamp through a bathroom door after a loud argument.
Nel mocked Pistorius' emotional testimony which was repeatedly interrupted by loud sobs and uncontrolled crying.
"We cannot argue that he was the worst witness ever, that honor belongs to someone else," Nel said in his written argument. "The accused was, however, demonstrably one of the worst witnesses ever encountered."
He said Pistorius' crying on the witness stand were "well-calculated and rehearsed emotional outbursts to deflect the attention and avoid having to answer questions."
The prosecution is focusing on 13 incongruities it identified in Pistorius' testimony, Nel said in his 100 page written argument.
Pistorius' lawyer, Barry Roux, will wrap up the trial Friday when he makes his oral argument.
Then it’s up to Judge Thokozile Masipa, who, along with the help of two legal assistants, will determine the athlete’s fate. South Africa does not use juries in trials.
Pistorius, who also faces gun charges, could be sentenced to at least 25 years in prison if found guilty of murder.
The trial, running intermittently since March 3 at the High Court in Pretoria, has featured testimony from mental health officials, ballistics experts, friends, managers, neighbors and an ex-girlfriend of Pistorius.
The trial has been marked by the athlete’s emotions, from deep, wailing sobs to vomiting as Steenkamp’s injuries were discussed. He's also studied a religious text and fingered rosary beads during the trial.
In addition, he was examined by a battery of mental health experts who said he is suffering from depression and raised the possibility of him being suicidal.
Prior to the shooting, Pistorius was seen as an international inspiration, an Olympian known as “Blade Runner” for his prosthetic legs.
Pistorius has pleaded not guilty. If convicted, he could face 25 years to life behind bars.