During closing arguments, Gerrie Nel also harshly criticised the legal team of the double-amputee, saying it floated more than one theory about what happened on the night he shot Reeva Steenkamp through a closed toilet door in his home.
He said defence lawyers had argued that Pistorius acted in self-defence, fearing an intruder was in the house, but also raised the possibility that he was not criminally responsible, accidentally shooting Ms Steenkamp because he was "startled".
"It's two defences that you can never reconcile," Mr Nel said as Pistorius sat behind him in the dock. The once-celebrated athlete appeared calm, in contrast to some past occasions during which he retched and wailed in apparent distress.
The fathers of the Olympic runner and Ms Steenkamp, a model and television personality, were in the Pretoria courthouse for the first time since the trial began in early March.
They sat at opposite ends of a long bench in the gallery. Pistorius is said to be estranged from his father, Henke, and Ms Steenkamp's father, Barry, has been ill.
Pistorius's older brother Carl, who has regularly attended court sessions, is in an intensive care unit in a South African hospital after a serious car crash last week.
Barry Roux, the chief defence lawyer, listened and checked files as Mr Nel spoke for hours, occasionally urged by Judge Thokozile Masipa to speed up.
Mr Roux will present his final arguments today before the judge adjourns the trial to deliberate with two assistants on a verdict.
The prosecution has argued that Pistorius intentionally shot Ms Steenkamp before dawn on February 14 last year after a quarrel. The defence has contended that he fired by mistake, thinking he was about to be attacked by an assailant in the toilet and that Ms Steenkamp was in the bedroom.
Mr Nel said it was improbable that the athlete rushed with his gun to investigate a purported sound in the bathroom without first trying to talk to Ms Steenkamp and confirm she was safe.
"We cannot argue that he was the worst witness ever, that honour belongs to someone else," said the prosecution's written argument. "The accused was, however, demonstrably one of the worst witnesses ever encountered."
The prosecution said Pistorius "used well-calculated and rehearsed emotional outbursts to deflect the attention and avoid having to answer questions".
A psychologist who examined Pistorius during a court-ordered observation period concluded he was severely traumatised and could become an increasing suicide risk unless he continued to receive mental health care.
In addition to the murder charge, Pistorius faces three separate gun-related charges. He has pleaded not guilty to them all.
Because South Africa has no trial by jury, Judge Masipa will decide whether Pistorius committed murder, is guilty of a negligent killing, or if he made a tragic error and should be acquitted.
The runner faces 25 years to life in prison if convicted of premeditated murder, and would also be sent to prison for years if found guilty of murder without premeditation or culpable homicide.
Mr Roux began his final arguments before Judge Masipa postponed proceedings until today.
He noted some of the evidence at the scene, which Mr Nel referred to, had been moved around by investigators. "We're not talking about a conspiracy," Mr Roux said, calling it unintentional.
He also noted that Hilton Botha, the former chief investigating officer in the case, had acknowledged mistakes in police procedure but was not called by the state as a trial witness.