why the HECK didn't he turn the light on?
I can kind of see it in a hurry of activity and high stress, and the possibility that there was actually an intruder, or even more than one intruder. If he thought there was someone in the bedroom, then he'd leave the light off.
But he says he was looking for his socks when he was putting his prosthetic on. In a pitch dark room.
And so much of this hinges on who he thought was in the toilet. If he still thought intruder, then the light (might) stay off. If he thought Reeva, then it could go on.
This is similar to why Nel is chasing on the 'checked the balcony' thing. He was out there either because he thought Reeva was in the toilet and he needed to shout for help, or because he thought an intruder was in the toilet and he needed to check for Reeva.
I suppose, a lot of this is hanging on the precise moment when Pistorias knew it was Reeva in the toilet.
I think this is why Nel is trying to pin it down.
Having said that, I'm now well back on the fence. I can imagine in that scenario, it wasn't a sudden lightbulb moment, but that he was wavering between 'it might be' and 'I hope it's not' and back again lots of times. That might explain why he's baffled by the questions (not in an understanding way, just in the way he can't pin an answer down).