Well all my family in SA live behind security, with armed guards.
All my SA friends have installed lockable wire mesh screens in their homes, on the landings.
They lock these every night, and even when they work at home.
So, for me, it's entirely plausible that he shot her thinking it was an intruder.
She may have locked the loo door out of habit for her own security, or because she wanted a private poo.
The same social context as the UK should not be applied. SA is a very violent country, and the murder rate is really high. Violent crime is so commonplace.
And let's not forget that OP is disabled [edited by MNHQ], with reduced mobility - literally a sitting duck. He may well have been half asleep, or had a few glasses of wine.
Not excusing him, but keeping it all in context, I don't think this was a domestic violence murder.
And I feel sorry for them all. Especially the families. As far as I can see, everyone is broken hearted.