Piecing together what I've read from supposed crew member comments (annoymorous, of course):
* HGR was on just her second movie as Armorer, scant experience;
* Neither guns used as props nor ammo - real and blanks - were properly secured;
* Crew members had been slightly offsite "plinking" with that gun just hours before the rehearsal in which the cinematographer was shot;
* Normal Hollyweird procedure is for the Armorer to load props with blanks or real ammo or nothing, as required for a scene; the First Assistant Director then inspects the gun, and tells the actor using it whether it is "hot" (loaded with something) or "cold"; actors rely on that double control;
* The First Assistant Director told Baldwin the gun was "cold".
In the direct sense, the killing was attributable to the armorer and First Assistant Director not doing their jobs: HGR did not secure guns and ammo properly; HGR did not unload the gun immediately before the fatal rehearsal, but much earlier (my guess); the First Assistant Director did not check the gun, taking the Armorer's word for it that it was not loaded; the First Assistant Director told Baldwin incorrect information.
That does not mean Baldwin is off the hook, except, possibly, criminally. From a civil law POV: he knew HGR was inexperienced; he knew (or should have) that at her previous gig her handling/security had been sloppy; he knew there had been three accidents on the "Rust" set with guns used as props, but did not can HGR; he knew (or should have) that the First Assistant Director had a reputation for sloppiness concerning safety; he knew (or should have) that the crew had been messing around with guns and ammo used as props. Whether that degree of negligence is criminally prosecutable, I won't guess. But on the civil side he should be expending millions of $$ in legal expenses and judgments plus years in court.
Baldwin's Hollyweird career should be over, but I don't trust Hollyweird to give a @#$% about the moronicity of one of its darlings.
