A bit more on requirements for reasonable grounds for subjective belief of lawful intent in PPD:
http://www.saflii.org/za/cases/ZANWHC/2005/105.html
[36] With regard to putative self-defence, Snyman in Criminal Law, Fourth Edition on page 101 states the following:-
“No ground of justification can exist in the absence of objective factors, and for this reason X’s conduct remains unlawful if she subjectively thinks that there is a ground for justification whereas in fact there is none. A so-called “putative ground of justification” is therefore in fact no ground of justification.”
[37] Jonathan Burchell in Principles of Criminal Law, Third Edition on page 243 states as follows:-
“A distinction must be drawn between private defence as a defence excluding unlawfulness, which is judged objectively, and ‘putative’ or ‘supposed’ private defence which relates to the mental state of the accused.
Where the accused raises the defence of private defence, the judicial inquiry commences with an examination of the unlawfulness or the lawfulness of the accused’s conduct. If the conduct is lawful, then an acquittal results. If the conduct is unlawful then the inquiry might not end there. Provided a foundation is laid for ‘putative’ private defence, then the court proceeds to examine whether the accused genuinely, albeit mistakenly, believed that he or she was acting in lawful private defence (where the charge requires intention to be proved) or whether this belief was also held on reasonable grounds (where negligence is sufficient for liability).”
http://www.csvr.org.za/wits/papers/papluds2.htm
The court must consider reasonableness based on what the accused knows and sees.55 For self-defence the court must establish that the accused believed she was acting lawfully, a subjective element, and that the circumstances justify that belief, an objective element. In determining the reasonableness of the accused's behaviour:56
Honest Belief
The most important requirement of putative private defence is that the accused honestly believed s/he was acting in private defence. An accused who is aware that s/he is acting unlawfully, or foresees that s/he could be acting unlawfully, including by being aware or foreseeing that s/he may be exceeding the bounds of private defence, cannot meet the elements of this defence and can be convicted of murder.109
The test for determining whether an accused held an honest belief s/he was acting in private defence is subjective.110 According to the criminal law experts, however, in testing the credibility of the accused, and therefore his/her honest belief, the court will consider whether a reasonable person would have held that belief. The more unreasonable the accused's claimed honest belief, the less likely the court will consider him/her to be credible.