The problems with your scenario begin right away. First, has "Character A" ever been convicted of a violent crime? What official history of "overreacting" do you refer to? Is there something wrong with being armed? Then why is it pertinent?
Character Z (by which of course I mean zed): No criminal record; suspended for tardiness; Skittles
If Character Z is a juvenile, then you have no idea if he has a record. Your statement that he is "suspended for tardiness" and "skittles" is trying to be cutesy and influence the perception of the reader with meaningless information.
The only reasons that A suspects Z of wrongdoing is that Z is wearing a hoodie.
Not true.
A, subsequently exits his car and proceeds to follow Z on foot, despite the 911 operator’s instruction that A “doesn’t need to do that”.
There was no "instruction" not to follow.
1) Who, if anyone, is morally responsible for Z’s death?
Well, we don't in this country have a formal mechanism for determining such a thing. Moral responsibility would be up to the reader to decide, based on facts rather than weasel phrases like "skittles".
2) Who, if anyone, is legally responsible for Z’s death?
You haven't provided enough information here to make a determination, unless you expect us to find Character A criminally liable because the other guy was just a kid skipping and whistling with skittles in his pocket.
3) What is the obligation of the local police force in investigating this incident?
To conduct a thorough investigation, WHICH THEY HAVE DONE.
Character A has been arrested for assaulting a police officer. Even if he hasn’t been convicted he is the only one of the two, at least as far as we know, with any kind of criminal history. You’re right that there’s no way to know whether character B has any juvenile record but there’s no reason to suspect that he does.
The reason that A being armed is important is quite simply that Z was not. When an unarmed Z ends up dead after being followed by an armed A, it’s very difficult for A to claim self defense.
And I was just trying to be funny with the Skittles comment. As for the “tardiness”, I’m sure I’ve seen several FReepers mention that that’s what he was suspended for. It’s the only disciplinary problem he’s had that we know of.