BIB I don't believe this is quite correct.
My understanding on this is that PPD is a legitimate defence to murder and if the judge finds OP's claim of PPD as valid, that is the end of the case. There is no murder charge, no CH and OP is innocent of all charges. The reason why is PPD is the lawful killing to defend yourself or property whereas murder is the unlawful killing of a person.
Here is a short excerpt from
http://criminallawza.net/2014/04/13/pistoriuss-new-defence/
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.