I still can't get past the idea that it is in any way plausible that your automatic assumption wouldn't be that the strange person in the bathroom would be, you know, THE PERSON WHO LIVES WITH YOU. No one suggested that OP had a history of sleepwalking or nighttime hallucinations. Masipa rejected the heightened anxiety claim, and he wasn't found to be mentally incapacitated in any way at the time of the shooting. If he had time to get his gun and position himself to fire, he had time to check the bed and realize Reeva wasn't there.
This isn't just a case of mistaken but honest fear. That is something you might claim if you shot someone who approached you while you were walking on a dark street at night, and made a gesture you perceived as aggressive. In that case, we might still decide that the fear wasn't reasonable enough to justify the response, but it would be somewhere in the realm of comprehensible human behavior. But if you can legitimately claim that you were afraid because you heard the toilet flush EVEN THOUGH you already knew there were others in the house, then we're living in a very dangerous world.
And from what I've read about domestic violence, the fact that OP was apparently devastated afterward doesn't prove anything. Men who kill their partners don't usually plan it out in advance, they do it in a moment of rage and don't think about the consequences until it is too late. That doesn't mean firing four bullets into someone isn't intentional murder in that moment.