The onus is always on the State even in PPD cases. It's true that Oscar had to produce a version that was reasonably possibly true and that explained his actions but that doesn't shift the burden of proof to him. You are confusing two things: the fact that without any defense there would be no basis for a court to conclude that he was acting in PPD as this is judged subjectively (and he is the only one who can testify to his state of mind at the time) versus a shifting away from the State to the defense to prove their case beyond reasonable doubt. You can see that the onus has not shifted because Oscar's version is accepted as long as it is reasonably possibly true - it does not have to be true beyond reasonable doubt.
Once Oscar has produced a reasonably possibly true account that the court accepts to explain why he needed to act as he did, then the court looks at his conduct in the light of that version. They look at it to see to what extent they can accept the aspects that are relevant to the alleged offence - as Judge Masipa did when she concluded that Oscar couldn't have had no intention to shoot when he picked up the gun - and make their judgement based on the elements of that account.
I agree. But this is completely different from your previous assertion that the onus was on the State to disprove Oscar's (self stated) state of mind at the time of the killing. He said he was terrified of attack, the State failed to prove that this was completely false due to him knowingly killing Reeva. However, they did establish that this was an irrational fear as there was no threat. The Defence failed to convince the court that this irrational fear had some reasonable basis behind it (disability, anxiety, "startle" nonsense) which leaves us with Oscar supposedly feeling terrified, the knowledge that this was irrational and your belief that Oscar's self reported emotions needed to be disproved by the State. It also leave us with a verdict that he acted unreasonably and with excessive force with, of course, the debate regarding negligence versus murder to come at appeal.
My question is how can you "disprove" what someone says they felt?
My views are that this is pretty much :
a) impossible without mind reading
b) rendered more difficult by his evasive, contradictory testimony and his "many thoughts"
c) open to retrospective interpretation. At the time of the shooting, or just before, the massive stress response provoked would have flooded his body with stress hormones, making it, in my opinion, hard to distinguish between rage or terror or which would then need to be assigned retrospectively. And, what's to say, that in both scenarios - murdering Reeva and killing an intruder - he didn't simultaneously
feel rage, terror (and maybe exhilaration??)