You might re-read Sherbert's post # 601 which quotes an earlier post of mine where I was sorting through these issues. I had been hung up thinking of "knowledge" of lawfulness as the kind of factual knowledge that he would have demonstrated on his firearms exam. But yes, your approach is more correct. See also Mr. Jitty's posts #647 & 649 above.
Not sure though how anyone could believe he would have thought he was acting lawfully firing 4 lethal rounds at an unidentified person through a closed door. He went to confront a supposed intruder, he located someone in his toilet, he aimed, he fired, and he killed someone before bothering to ascertain if they even were an intruder. He was unable to justify his actions as a lawful PPD response.
I don't see it that way though...
If, on his version, he was sure Reeva was in the bedroom with him, then the only possible explanation for the noise he heard was an intruder opening the window - which was then confirmed (in his mind) when he heard the door slam and saw the window open. If he genuinely believed Reeva to be in the bedroom, who else- other than an intruder - could have opened the window and made the door slam? So as far as he was concerned, he had confirmed the presence of an intruder. On hearing the wood moving noise he believed the door was opening. (on his version he did not know it was locked at that time). This - again on his version- he interpreted as an attack beginning, panicked and fired - at a door he believed to be opening and to defend against an attack he believed was beginning.
If the door had opened and an armed intruder had begun attacking him, he would have had a lawful PD defence. If he had acted as he did (firing through the door before it actually opened), and an armed intruder had been shot and killed, it wouldn't have been a lawful PD response as he acted too hastily, but if he had genuinely believed the attack was beginning, it would -imo- have been found as CH, as there would not have been an unlawful intent.
As far as I understand it, PPD would always be unlawful, because of the putative element