Yes, I thought this was really interesting. There was some debate as to whether Nel should have objected or whether he would mention it in Chambers at some point.
I find Judge Greenland to be fascinating to listen to. He has tremendous clarity of thought and a wealth of knowledge and experience to draw on.
I can't see the admissibility problem. At the very least the words quoted appear to imply that OP thought at the time that it was OK deliberately to shoot putative intruders dead in the absence of any signs of aggression on their part, without either seeing them or hearing them speak.
They also show (what everyone knows) that OP had not yet invented the panic-deprived-me-of-responsibility-for-my-actions idea. He had seized on the self-defence notion as a good hope. He hadn't yet realized that you can be convicted of murdering even an intruder and he hadn't yet spotted the idea of denying any volition of any form.