Sorry if it has been discussed already, but why would it matter whether Baldwin did or did not pull the trigger? Wasn't he expected to do just that for realistic filming purposes, to point the gun and pull the trigger?
Gun safety rules, whether on or off film sets, prohibit pointing the gun at a person, and doubly so while pulling the trigger. On sets, the procedure is to use camera angles to create the appearance of pointing, but with the gun actually pointed in a safe direction. If it must be pointed at the camera, as in this case, then either the camera is remote controlled, or a transparent shield is provided.
Since this was a rehearsal with a cold gun, the procedure was to aim just to the side of the camera and the two people behind it. Then the gun was to be cocked by Baldwin, as part of the scene.
This all went as planned, but after the scene, Baldwin went to de-cock the gun, and in so doing, the aim shifted slightly and the gun went off.
How exactly that happened is not known. Baldwin said he lowered the hammer and the gun discharged. But for that gun, the hammer cannot be lowered without pulling the trigger. The procedure is to put your thumb on the hammer, pull it back, squeeze the trigger, and lower the hammer.
The truth is for that type of gun, accidental discharges are common during de-cocking. All it takes is a thumb slip on the hammer. In fact that had already happened in an outdoor scene with a similar gun, loaded with blanks. So the procedure is always to point the gun down and away when de-cocking.
When Baldwin says he didn't pull the trigger, I suspect he means he didn't pull the trigger to fire the gun. But he had to have held it to lower the hammer. The other possibility, is that he held the trigger the entire time, and thus did not cock the gun, but instead held the hammer back with his thumb. That again is something gun safety would never allow, because of the high risk of discharge.
Any way you slice it, Baldwin did not follow gun safety rules, and they appear to have been lax overall on the set. Still it was an accident, and not intentional. But it should never have happened. Safety protocols broke down at every level, from the armorer to assistant director to the actor. None of them followed the rules.