He seems to be a very anxious individual. I have not heard any more about the steroids he had been taking, but they were mentioned some time ago, and would have affected his attitudes. S A is a violent country, and people have guns to protect themselves from intruders. Add that to his personality,and you get a rather trigger-happy man in a culture where using a gun is a less extreme concept than it is here.
He should have checked that his girl-friend was still in bed. He should have called the authorities and not dealt with it himself.
If it had not been for the earlier disagreement with his girl-friend, I would have gone along with his story. I suspect he meant to frighten her, but she was in line with the shot and was killed. He was beside himself when the authorities turned up - was that because he realised what he had accidentally done, or because he had cooled down and realised that he was guilty of murdering her?
He has not been a good witness, by all accounts, having an eye more on his testimony showing him in a good light than on the absolute truth, and contradicting himself.