Self-Defence Not Much Of A Case For Pistorius

Not sure whether the shooting of Reeva Steenkamp by Oscar Pistorius was tragic accident or murder and we won't find out for a few more months yet as the wheels of justice in South Africa seemingly turn very slowly.
From the parts of the proceedings that i have heard so far, the Pistorius case sounds extremely flimsy so i am fully expecting the prosecutor to unpick his chain of events. 
Already there are questions over why neighbours reporting the sound of shouting and screaming before hearing the gunfire, how the gun's holster was found under the side of the bed where Steenkamp was sleeping so Pistorius would have seen she wasn't there, how the angle of the shots fired through the door show that he fired down and was therefore wearing his artificial legs while Pistorius says he did not put on the prosthetics and why he told the guards at the gated estate that everything was 'all right' when they called to check after reports of gunfire at the house.
What i don't understand is the defence that he thought it was a burglar locked in the toilet which is why he shot four times into the bathroom.
If the 'burglar' was locked in the toilet and he was outside armed with a powerful weapon, why not phone the guard or the police? If Pistorius wanted a show of force' to show the 'burglar' that he really was out there with a gun, shoot into the floor or wall, not into the room where he or she was.
If it wasn't his girlfriend on the other side, if it was someone intent on ransacking the house, he still would have killed someone that he didn't need to kill.
Surely self-defence can't be a just cause for murder or manslaughter if the person you are supposedly defending yourself against has locked themselves inside a toilet and you are outside with a gun.