LONDON (AFP) - South African star sprinter Oscar Pistorius has gone from being a hero to a devil, Ms Reeva Steenkamp's mother June said in a British newspaper interview published Thursday.

She told the Daily Mirror she is obsessed with watching Pistorius's every move in the Pretoria courtroom where he is being tried for the killing of her daughter, a law graduate, model and aspiring actress, on Valentine's Day in 2013.

Prosecutors say the 27-year-old Paralympian and Olympian deliberately shot Ms Steenkamp in the early hours of Valentine's Day after the couple had a fight.

He says he shot her by accident through a locked bathroom door after mistaking her for an intruder.

"I look at Oscar the whole time, to see how he is coping, how he is behaving. I'm obsessed with looking at him, it's just instinctive, I can't explain it," Ms June Steenkamp told the British tabloid.

"I keep thinking: 'let me see how he's taking this'. He has been very dramatic, the vomiting and crying.

"I think he's just about keeping himself together. I don't know whether he's acting."

Ms June Steenkamp has never met Pistorius. The double amputee did not acknowledge her on the first day of the trial but did eventually look at her and say good morning.

"My presence unnerves him, I'm sure of it. He's answerable to me," she said.

"He must see me there in the court, he must feel my eyes boring into him, I think it makes a lot of difference.

"I do look at him too much, maybe. I like to see how he is reacting. I can see him very clearly, even without my glasses - he's the only one I can see, he's right in front of me.

"He has an aggressive persona, he's used to having people adore him, so it must be pretty different for him now.

"He's been spoilt by other people, that's why he struts around and looks superior.

"He's gone from hero to devil."

Ms June Steenkamp, who has remained stoney-faced throughout much of the testimony during the trial, said she does not want to cry in court but when she returns to her hotel room each day "I start crying out, crying all the tears and pain I've held inside."

She said that particular images seen during the trial would remain with her until her dying day.

The 67-year-old said attending the trial was "hell" but she was "compelled to be there".

Her husband Barry is also determined to attend but has been too ill so far after suffering a stroke.

She said she did not care whether the Paralympics multiple gold medallist was convicted or not.

"He has to stand up to what he's done and - if he has to - pay for it," she told the Mirror.

"What difference is it going to make to me if he goes to prison for 25 years or is allowed to walk free? "I'm not a person who wants to punish him. I want my daughter back, but it's never going to happen.

"She was the most wonderful, beautiful person, inside and out, and she was everything to Barry and me - our lives are destroyed."