I think his own admissions are enough for a conviction of culpable homicide. If he does not testify, then the case for culpable homicide is even stronger.
If he testifies, then I think there is a chance that he could be acquitted if he can persuade the judge that he really did believe that an intruder was in the bathroom and he feared an immediate physical attack. Even if she accepts his version of mistakenly believing there was a burglar, there is still the question of whether his reaction was reasonable, given his mistaken belief. On that I have no idea what the judge will find - I think a lot will depend on how credible his testimony and his description of the circumstances that led to his fears and his use of lethal force.
I've been reading up a little on SA self defense and justification, and here's an article that explains it a little bit:
Killer rugby star Rudi Vleis Visagies case may aid Oscar Pistorius in defence
I have also read some caselaw, and there are varying opinions about when homicide is justified by self- defense (or "putative self defense"). As I suspected, it is not black and white, but it is a subjective weighing of the circumstances. So it will come down to the discretion of the judge.
I've also found that in SA, cases get reversed pretty regularly - it is not like the appellate system we have in the US.