hi dig,
i agree that you caveated!
[my further comments in brackets]
As far as a dutiful neighborhood watch, my understanding is he made a call to authorities (a good NW thing to do) [agreed]
was told to not pursue, [actually, he was told the police didn't need him to pursue M, which isn't quite the same, but anyway, assuming cessation was the police's intent ...]
decided his judgement to be better than professional instructions [this is presumably the prosecution's argument but the defence denies it and only z knows his own mind]
and pursued (disregarding expert legal authority, less than stellar NW behavior) [Z says he followed police advice and was returning to his car].
To insert himself at this point is, from my tabloid perch, is meddling and up to no good. [Yes, but the insertion/meddling model is tabloid speculation for which there is no eyewitness testimony or actual evidence. Z's model that M was hiding and emerged from hiding as he returned to his car is not implausible to me]
Since I said almost maniacal, I suppose that means he wasn't maniacal, the distance from that which constitutes "almost" a matter of ones own value interpretations. [okay. But I think a different state of mind is much more likely. These two folks each feared the other. They ended up in a fight scenario. One of them had a gun. At this point, I think you agree that the law permits Z's actions. But if we agree Z was fearful of someone whom he sincerely believed to be scoping houses for the purpose of burglary, and then fortune arranged an accidental meeting between him and "the suspect", he's not such a bad egg. And that looks to me like what happened. Plausibly. And I think probably. It's certainly a scenario that is sufficient to create a reasonable doubt, which is the standard.]
By the way, you should read carefully the self-defence standard which clo provided. Z clearly falls within it.