I am edging towards guilty.
I've been on the fence for so long that it feels strange, but I am now at 'guilty'.
I have to admit, I'm not fully sure of what he's guilty of, but I'm convinced that a) he shot at the door fully intending to kill who was inside and b) he had no reason to believe it wasn't Reeva in there. It's the first time I've thought of it from that angle.
The back to the bed, pitch dark and not getting a verbal response are the things that are swinging it for me. He's using it as an excuse to say that he didn't know she WASN'T in the bedroom. But with those three things together - he also had no reason to believe that Reeva WAS in the bedroom. He didn't see her - he didn't hear her. Just to say 'it was the last place I saw her' doesn't quite work for me. We're not talking about an inanimate object. He says himself that she was awake just minutes before.
Even if we were to suppose that the first two points were right - it was so dark and the fans were so noisy that he could not possibly have noticed her creeping away (and why would she creep away?), then you still have the stumbling block that he called in a low voice/whisper to call the police, and she didn't answer. If you were being silently ignored by someone you knew was awake minutes before, would you not repeat or call out the name or something to get a response?
So he didn't see or hear her in the bedroom. Why would he think she was there? It's not like there were no other possibilities where she could have been.
If they were alone in the house together (Frank excluded) and there was a noise from the bathroom, it almost stands to reason that as he did not know that she was in the bedroom, then the first thought would have been that it was her in the bathroom.
I suspect I've got some of this wrong and that my thinking is flawed - it often happens that way. It's just where my mind is at the moment.