Or pissed off at her, bad mouthing her to someone....
Motive...?
There was a meme right when it happened that said there was an exchange like the director saying cut or reshoot and Baldwin jokingly pointed the gun at her and said something like I aught to just shoot the both of you, and blamo, the gun had a bullet in it and worked perfectly.
Flippant careless comments and actions but manslaughter just the same.