His version is plausible, just.
I agree... to a point.
Even his version, he didn't do nearly enough to establish where Reeva was before shooting. He didn't get any kind of confirmation from her that she was still in the room, and without her going out the door, it was a relatively small area. He thought 'there's three of us in this area, and at least one gun...' but apparently did not check where Reeva was.
So that makes him guilty of the culpability one.
Secondly, for one of the murder levels to stick, all they have to show was that he had reason to believe that by shooting into the locked toilet, he expected whomever was in there to die. It doesn't matter who it was at that point - if he had reason to believe they would die, and he did not think his life was in immediate danger, then this is one of the murder charges (one of three levels).
He must have expected them to die from the type of bullet used and the size of the room. Equally, while he might have thought his life was in danger, he has nothing to back this up. He could not, because he did not see that a) it was a stranger and b) they were armed. If he thought hi was in danger, it was entirely the work of his imagination. 'I guessed, I supposed, I thought...' At the end of the day, he SAW nothing. That's not enough.