The issue is his actual state of mind at the time he fired his gun. If he genuinely, but mistakenly believed that an attack was imminent or his life was in danger, then he didn't have the necessary intent to commit an unlawful killing. There was no constructive intent (dolus eventualis.

Some quotes I assembled from various places:

Where the form of fault required for an offence is intention (such as for murder), the accused must intend to kill another human being, but also, to do so unlawfully – that is, s/he must know or foresee that s/he is unlawfully killing another human being.....As a result, an accused who is ignorant or mistaken as to the fact that s/he is killing another human being or that s/he is doing so unlawfully, has a valid and complete defence where the form of fault required is intention (murder).

and

In a case where the state has proven that an accused had shot and killed another person in his house in the absence of an attack on his life or property, it would be difficult to escape conviction for murder unless the accused is found to have acted in putative self-defence. Where an accused is found to have genuinely believed that his life was in danger and that he was using reasonable means to avert an attack on himself or his property, he may escape conviction for murder on the grounds that he lacked the requisite intention.

...As intention is tested subjectively, the pivotal question would be what the actual state of mind of the accused was at the time when he killed the victim. If an accused genuinely but mistakenly believed that an attack was imminent or that his life was in danger, the court will find that he lacked the intention to be convicted of murder.

Here's a 2011 widely cited case on dolus eventualis and a very recent one. From the latter:

The test for dolus eventualis is twofold: (a) Did the appellant subjectively foresee the possibility of the death of his passengers ensuing from his conduct; and (b) did he reconcile himself with that possibility…”.

...It is clear that the first element of dolus eventualis – subjective foresight of the possibility of death, as a result of his conduct, taking place – was present. Everything, then, turned on the second element. This element, said the Appeal court, had been explained by Jansen JA in S v Ngubane¹⁸, called the 'volitional element', which is present when the accused '''consents'' to the consequences foreseen as a possibility, he ''reconciles himself'' to it, he ''takes it into the bargain''.

There was no evidence Oscar contemplated that anyone other than the intruder was in the bathroom or that he might kill someone other than the intruder. Whether he should have known there was a possibility Reeva was in the bathroom is not the test. It's what he actually knew. Whether his belief was reasonable is the test for negligence, not intent.

In short, the state had to prove he intended to kill another human being, but also, he intended to do so unlawfully. If he never considered the possibility, then dolus eventualis shouldn't apply.

As to the weight the appeals court will give to the trial judge's ruling, here's a case on that:

In Rex v Dhlumayo 1948 (2) SA 677 (A) at 702, Davis AJA remarked that:

‘It would be most unsafe invariably to conclude that everything that is not mentioned [in a judgment] has been overlooked. … Lord Wright cites with apparent approval … the statement of Lord Buckmaster in Clarke’s case; [Clarke v Edinburgh and District Tramways Company (1919 S.C (H. L.), 35] with which Lord Atkinson had expressly associated himself, that

“Courts of appeal should not seek anxiously to discover reasons adverse to the conclusions of the learned Judge who has seen and heard the witnesses and determined the case on the comparison of their evidence.”’

Marais JA in S v Naidoo 2003 (1) SACR 347 (SCA) para 26 also emphasized the above quotation by saying the following:

‘In the final analysis, a Court of appeal does not overturn a trial Court’s findings of fact unless they are shown to be vitiated by material misdirections or are shown by the record to be wrong.’

I wouldn't call the judge's ruling a slam dunk for the state by any means.