You just described the very reason that the responsibility for weapons protocols on a film set are left to a professional armor who primary mission is to ensure that every weapon on set is certified and verified as a cold gun. Which is the industry term for a triple checked and certified unloaded weapon. Actors are some of the dumbest people on earth they are not qualified nor tasked with weapons protocols. The armor F’d up big time when she handed a loaded weapon to an actor and F’d up again when she then certified that as a cold gun. The improper handling of a historical weapon that lacks basic safety features is a whole separate issue. It could be argued that he lost his grip on the hammer and it slipped due to perspiration or hand oils off his thumb a modern revolver would have gone click and nothing else a historical one with a live firing pin under that hammer goes boom nearly every time. He was being directed in the positioning and pointing of that weapon by the photographer the industry term is angles so it will be hard to prove he intentionally pointed it when he was being instructed and positioned at the time. The liability for a cold weapon is on the armor the actor lacks the qualifications to certify that.
All that explanation sounds great, but it has little to do with NM statutes on the discharge of a firearm resulting in death. Baldwin faces (or should be facing, in a sane world where law treats everyone equally) that fact that under NM law, he discharged a firearm resulting in death and grave bodily harm.