"If somebody breaks into your house, you are entitled to kill that person under certain circumstances. If the burglar runs away and you kill that person anyway, you could be exceeding the bounds of self-defence. If, however, you are faced with a situation where you fear for your life or safety, you would be entitled to kill the intruder."
- no one broke into his house
- no one ran away
- no intruder made him fear for his life or safety
- Reeva was not an intruder
He was under (perhaps) a mistaken belief that A) there was an intruder; and B) that intruder could harm him.
I think in the UK** there would be some kind of test to see what a 'reasonable person' would believe in the same circumstances. And I think from the MN massive, the "reasonable MN'er" would have checked the whereabouts of the partner before going to the bathroom with a gun.
Also as he never saw an intruder, and presumably the 'noises' he heard were simply those of Reeva going to the toilet (ie fairly innocuous normal sounds), is it reasonable to experience the level of fear for his life of safety that he did under these circumstances?
* could be wrong - law school was years ago*
I think there is a possibility he might be faced with using some kind of mental illness defence, if he is going to insist he was so paranoid and frightened from the above.