The South African former Paralympian athlete, who is awaiting a jail sentence for murdering Steenkamp in 2013, told British broadcaster ITV that he did not kill his partner intentionally.
"I don't want to go back to jail. I don't want to have to waste my life sitting there," he said in footage released ahead of his first television interview, which will be screened on Friday.
"If I was afforded the opportunity of redemption, I would like to help the less fortunate," he said.
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"I would like to believe that if Reeva could look down upon me that she would want me to live that life."
In a case that drew worldwide attention, Pistorius, 29, shot Steenkamp in the early hours of Valentine's Day 2013, claiming he mistook her for a burglar when he fired four times through the door of his bedroom toilet.
He was found guilty of murder in March, after spending a year behind bars on his earlier conviction of the lesser crime of culpable homicide, the equivalent of manslaughter.
The Supreme Court of Appeal ruled that Pistorius was guilty of murder -- irrespective of who was behind the door when he opened fire with a high-calibre pistol he kept under his bed.
"Reeva was a fantastic person, but if (people believe) that I took her life intentionally, which has not been found, then it's a very sad thing," Pistorius said.
In the interview, he relived the night of the killing, saying "I did take Reeva's life and I have to live with that.
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"I can smell the blood. I can feel the warmness of it on my hands.
"I understand the pain people feel, that loved her and miss her. I feel that same pain. I feel that same hate for myself.
"I look back and I think, I always think -- how did this possibly happen?"
Last week, Pistorius was criticised by prosecution lawyers for apparently being too ill and depressed to face cross-examination at his sentencing hearing yet able to give the TV interview.
His sentence will be handed down by Pretoria High Court on July 6.
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