"What actually came out in court is not the truth," her father Barry Steenkamp told Australia's Channel Seven TV.
South Africa's justice minister blocked Pistorius' early release from prison, due last week, and thereby beclouding the athlete’s hope of securing a release with uncertainty.
The Paralympic star was jailed for five years in 2014 for culpable homicide, a charge equivalent to manslaughter.
The Olympic athlete admits shooting dead Ms Steenkamp, but insists he mistook her for an intruder.
But this was dismissed by Mr Steenkamp: "He got angry, she went off to the toilet, locked herself inside, and then him pulling out the gun and shooting."
"Why didn't he just let her walk away?" her mother June Steenkamp said, reiterating the family's belief that Pistorius killed Ms Steenkamp deliberately.
Justice Minister Michael Masutha last week said that the decision to free the athlete after serving 10 months of his five-year sentence had been taken prematurely.
He has now suspended the decision and sent it back to the parole board for review, a process which could take months, legal sources have told the BBC.
Under South African law, Pistorius is eligible for release under "correctional supervision", having served a sixth of his sentence.
Prosecutors have lodged an appeal, calling for the athlete's conviction to be converted to murder, which carries a minimum sentence of 15 years.
During sentencing, Judge Thokozile Masipa said the state had failed to prove Pistorius' intent to kill when he fired.
