PRETORIA, South Africa (CNN) -

Did Oscar Pistorius have time to think before he shot and killed his girlfriend Reeva Steenkamp last year? That's the question that state prosecutor Gerrie Nel dug into Thursday with a doctor who has spent years working with the disabled athlete.

"His intention was to shoot, whoever he came across -- if he came across anyone" in his home on a February night last year, Nel put to the doctor, Wayne Derman.

"I suppose if he came across an intruder or danger, he would shoot," Derman responded.

And Pistorius was firing to kill, the prosecutor said.

Derman, a defense witness, paused a long time before he finally responded.

"He was aiming to neutralize the threat," the doctor said.

Nel let those words hang in the air for a long moment before announcing that he was done with his questions for the day.

Pistorius, 27, is on trial for murder, and while he admits firing the bullets that killed Steenkamp, he pleaded not guilty. He says he mistakenly thought he was defending himself from an intruder.

The prosecution says the two had an argument and he deliberately murdered the model and law school graduate, who was 29.

'Two Oscars'

Derman spent all day Thursday on the stand, first testifying under defense questioning that Pistorius' double amputation is key to understanding his behavior.

Do not be taken in by the fact that Pistorius is one of the fastest runners in the world -- remember he is disabled, he argued.

Derman ran through a long list of the difficulties that double amputees experience every day, concluding: "The saddest thing I have learned through my six years of working with athletes with disability is that disability never sleeps.

"It's there when you go to sleep at night and it's there when you wake up in the morning. It affects nearly every aspect of your life," he said.

Pistorius' defense team seems to be trying to establish that he acted reasonably, given who he is, when he killed Steenkamp in the early hours of Valentine's Day 2013.

Judge Thokozile Masipa must decide whether he made a genuine mistake and, if so, whether the mistake and his response were reasonable.

The South African Olympian's defense team has been exploring his psychology this week.

His lawyer Kenny Oldwadge posited Thursday that there were "two Oscars," one of whom was a global sports star and one of whom was "vulnerable" and "scared."

"I am stuffed without my legs on," his lawyer quoted him as saying, using a slang term meaning "in trouble."

Witness reliability

Prosecutor Nel went after Derman aggressively in his cross-examination, exploring whether the shooting was an instinctive "fight-or-flight" response or the result of conscious thought.

The question of whether Pistorius is found guilty of premeditated murder could hinge on the answer.

Exchanges between the two got testy, with Nel trying to get the doctor to explain Pistorius' actions moment by moment on the night he killed Steenkamp.

Derman said it was impossible for him to answer theoretical questions about what Pistorius might have been thinking or doing at a given instant because he, Derman, knew what had actually happened after the moment in question.

Frustrated, he appealed to Judge Masipa, but she sided with Nel, firmly instructing the witness to answer his questions or say he didn't know.

Nel began his cross-examination by suggesting that Derman was not a trustworthy witness because he had a responsibility to Pistorius as his doctor. Masipa rejected the objection.

The prosecutor kept up the attack, demanding to know when Derman had made particular notes on Pistorius, whether the court should believe Derman or Pistorius when their evidence differed, and the meaning of the word "subsequent."

Derman grew irritated with Nel when the prosecutor responded: "That's not true" to a statement Derman gave, and he appealed to the judge that Nel was being "unprofessional."

Masipa agreed and ordered Nel to withdraw the remark.

Nel, in turn, showed annoyance with Derman asking which "incident" he was referring to.