How can anyone see this interview as anything other than an attempt to sway the sentencing? As you rightly say, this should be seen as an aggravating factor, and actually increase rather than reduce what should already be a substantial sentence. Cynical and manipulative is an obvious description.
How is this even legal? How does a convicted murderer get the right to give an interview at all, let alone on the eve of sentencing?
The interview is not due to transmit until tomorrow (Friday) evening, which will be after his sentencing is announced. So the content of his interview is unlikely to be an attempt to influence the sentencing itself. On the other hand, I suppose there are a small number of possible motives for Pistorius to have conducted this ITV interview:
1) A factually-innocent (of murder) Pistorius is making one last public attempt to get his story out there - possibly with an eye to future appeals down the road.
2) Pistorius makes a full "mea culpa" confession, states that he knew it was Steenkamp behind the door, and that he fired at her through the door in a blind rage, and that all he wants to do now is beg forgiveness from Steenkamp's family and mercy from the courts and the public.
3) Pistorius accepts that he's guilty of murder, but wants one last public chance to convince the world that it was never Steenkamp that was his target, and that he truly did believe he was firing at the mythical intruder.
4) Pistorius does not accept that he's guilty of murder, he intends to communicate his belief that his actions were entirely justified and that he was acting lawfully to protect himself and Steenkamp (by firing lawfully at the mythical intruder). The interview will thus be used as an attempt by Pistorius to influence public opinion towards him in the longer term.
5) Pistorius is doing the interview purely for the fee he might be paid for doing so - he's reportedly broke.
Of course, there may be more than these four possible motives, but I think these are probably the most obvious possibilities.
I'd rule out (1), as I think there's more than enough solid, reliable evidence to safely convict Pistorius of murder (though the verdict is agnostic about whether Pistorius believed he was firing at Steenkamp or the mythical intruder, and the verdict does not need to be specific on this matter to be valid).
I'd also rule out (5), since I doubt ITV would make direct financial payment to Pistorius a significant part of the contract - ITV realises that it would look extremely bad for a broadcaster to be paying a convicted murderer large sums of money for an interview. In any case, the sums of money we're talking about would likely make little dent in Pistorius' financial position.
So now we come to the three more reasonable (IMO) possibilities: (2), (3) and (4).
I rule out (2), because I don't think that under any circumstances Pistorius will ever state that he knew it was Steenkamp behind that toilet door. I make that judgement on his acts and testimony since that fateful day and throughout the trial process, and on a wider evaluation of Pistorius' personality. In addition, any statement on Pistorius' part that he knew it was Steenkamp behind the door would also destroy all the self-defence aspects of his version of events, and therefore would effectively amount to a confession of murder.
I rule out (3), because I don't think Pistorius will EVER concede that he acted unlawfully and illegally that night. Whether Pistorius does or does not believe in his heart that he acted within the law that night (and IMO he knew he was firing at Steenkamp, ergo he knows in his heart that he committed a grave criminal offence that night), I think he long ago calculated that he could and should stick to his story of genuinely believing that Steenkamp was lying in bed, and that there was a potentially armed and dangerous intruder literally on the point of placing him (Pistorius and Steenkamp) in mortal danger.
So that leaves (4). I think this is by far the most likely motive for Pistorius giving this interview to ITV. I think the narcissist in him wants one last spot in the limelight, and one last chance to try to convince the public that the courts have got it all wrong and that he actually did nothing at all illegal that night. I think his chief concern is his reputation and his legacy, and possibly his position in society when he finally does get released from prison.
Let's hope Masipa can do a competent job under the legislation and procedural codes that she is obliged to follow scrupulously, and thus apply the correct and just sentence to Pistorius tomorrow. We'll see.