You are entitled (justified) in law to intentionally lawfully (such as, in self-defence) kill another human being. There is no criminal liability for doing so. Whether you are under attack and entitled to act in self defence is judged objectively, on the facts.
Pistorius is not making this claim. He accepts he acted unlawfully – he was not entitled to shoot at anyone that night.
In addition, you will be excused on a murder charge for unintentionally unlawfully killing another human being. This is a mental state defence – it denies the required guilty mental state.
For murder, you must intend to unlawfully kill. If you are mistaken, and genuinely believe you are acting lawfully (such as in private-defence (the technical name for the defence under which self-defence is located)), whereas you are not acting lawfully, you cannot be convicted of murder because you don’t intend to act unlawfully. To escape a conviction of culpable homicide this mistake must be reasonable – one which the reasonable person may make. But on a murder charge, it is enough, for an acquittal, if the accused was subjectively mistaken.