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