As to how the trigger was pulled, we know from his history that Baldwin is not a careful person.
He has showed a lack of discipline and forethought in his life numerous times.
We also know that his past includes heavy alcohol and drug use and while he might have been sober at the time of the shooting it's possible that his use long-term use of drugs and alcohol lessened his motor control or caused spasms.
It is easy to imagine that he was having a conversation with the victims and, as people do, he was gesticulating to emphasize points he was making. Unfortunately he was holding a gun that was able to launch something, somehow out of the muzzle.
So as he is waving his hands about, and maybe even using the gun to point directly at the victims, he carelessly put his finger on the trigger and his lack of motor control or a spasm caused him to pull the trigger.
This seems more plausible to me than him deliberately pointing a gun at someone and pulling the trigger on purpose.
He's still responsible, of course. Both as the person who did the shooting and as a producer.