Coming from someone who was born and raised (for the most part) in South Africa, let me say a few things:
Most South Africans live in fear; which is not to say it is paralyzing fear though for some people it is. Rather, it's a constant vigilance; a heightened sense of suspicion on all that happens around you. You learn to live with it. I have lived in 4 countries and in a community of people from the Americas, Asia, Oceania, Continental Europe and I can say that all of them have been totally in awe at some of my very typical stories which from my point of view are unexciting compared to some of my pals. Almost all my friends have so far been robbed in their homes (or serious attempts) or hijacked (I was fortunate to have neither). One friend was held semi-hostage and was tortured as an 8 year old. South Africa is very dangerous. And what is worse, the police are totally and utterly unreliable. Which is why we have private security companies. Seems a bit insane, but if you can afford it that is how you deal with robberies/criminals. You don't call the police because they will pitch up two hours later wiping the fried chicken off their cheeks, eventually losing all trails and possibly stealing from you. You call your security company who are armed response units that will do their best to track down criminals. I mention this to illustrate that South Africa is not like Europe/England/America. We are brought up to be suspicious. Criminals are ruthless. Living in a gated community means almost nothing. It does add a level of safety, but it only deters some opportunist thieves. Guards are easily bribed - that's all it takes for someone to gain access.
So what would I have thought in that situation? Here is my thought process putting myself in his position (not saying it is what he did since we don't know until his trial starts):
I wake up and hear suspicious noises. Check if wife is okay. She's not there. My assumption is not that she's the one making the noises. My assumption is that somehow they've gotten to her - either secretly in my very bed they've taken her, or perhaps she did in fact get up to go to the loo but they were waiting for her or it coincided with their time of attack. It's 3am so I can hardly think straight about it. I have no legs. Do I put them on and risk wasting time while they possibly kill my wife? Nope. I get my gun. I hear noises in the bathroom. I am scared. I panic. I shoot. I think anybody would've done that, really. Well, rather, it could happen to anybody and it isn't impossible to think that way. People from comfy European safety standards think it's insane to shoot first and to shoot lots. Yeah, well, the reality in SA is that if you don't shoot to kill or at least really do damage you're most likely going to be the one dead and your wife will end up raped and then murdered. Graphic but it's a repeated story in SA.
Why did Oscar not check if his girlfriend was there? Well if it's true that he didn't I can only say that he must have panicked as anybody would. Or just assumed she wasn't there.
Why did he not put on his legs? Why would he have to?
Why do the ballistics (apparently) not match up with the fact he didn't have his prosthetics on? Well, it isn't unthinkable that he shot from above his head, reaching his hands up to take a shot. Why would he do it, I dunno, but it isn't unreasonable.
The point is that at 3am in South Africa when you hear suspicious noises you don't sit down with a pen and paper with some gin and a cigar to plan out your attack. You don't think about calling the police. You only think about saving yourself and your loved ones. All this with a clouded mind too. I don't know if he is guilty or not of pre-meditated murder, God knows and may justice be done, but a lot of the things he did are not unreasonable especially within the context of living in South Africa.
The one thing I do question is that presumably he had a panic button in the room (most middle-class houses and upwards have these in bedrooms) and he didn't press it. Though again, the heat of the moment and panic clouds ones judgement.