There were previous AD/ND’s on set, as well as safety complaints. Those two facts dramatically change the circumstances from “freak movie set accident” to “pattern of unsafe conditions.”
The prosecutor, apparently, has communicated with other actors (not connected to Rust) who say they always check weapons themselves (which they should). And if you’re handed a gun on a set, as an actor, where there have already been accidental discharges, then any reasonable person would take extra precaution (not just trust a gun handed to you).