I have no idea how accurate it is, but IIRC, some of the early reporting was that people brought live ammo so they could go out and do target shooting as recreation. If that's true, and it was generally well-known on set, then that might weaken Baldwin's claim that he had no reason to check the gun because he's just an actor.
That is what makes me wonder why the director was not charged too, they have power on the set.
Some people have responsibility. Baldwin seems likely to be one,he pulled the trigger. But it seems his defense has a lot of others to point to.