They all played roles, but their level of responsibility differs. The armorer failed to do her job, but she didn’t pull the trigger. She can only be charged within the sphere of the consequences of her failing to do her job. Baldwin can’t be charged with negligence, because he didn’t prep the gun. But he can be charged with an offense for not checking the gun and for pulling the trigger.
The armorer is the most guilty. She was the ultimate person hired specifically to handle the weapons. If she did her job properly, there would have been nothing in the gun at that point in time. She gave her official ok to that weapon.
Baldwin should not be charged for not checking the gun. Actors are NEVER supposed to do that; that duty always falls to the armorer and his/her team. If an actor does anything with the firearm that isn’t called for in the script, the armorer should immediately take the gun, recheck it, then hand it back - with instructions to simply follow the script. According to what we currently know, Baldwin wasn’t supposed to squeeze the trigger in this particular scene, but he obviously did. That will have to be answered.
It sounds the opposite of everything the average person is taught about firearms, but on a movie set that’s how it works. There is a clear chain of command for the handling of firearms on a set. The armorer will also have much to answer for.