Normally, there is someone a film set who is responsible for prop firearms, including instruction of the actors in their use.
Seems like Alec Baldwin was not instructed. Of course, it isn’t hard to understand that even a prop firearm has explosive ammo in it, and that something, paper or plastic, might fly out. And he hit TWO people, killing one. Was it a big blank-loaded shotgun? They don’t say what it was or in what situation he fired it at them. It wasn’t during filming; he would not have been firing at directors during a scene.
Probably him being him. Clowning around.
As much as I dislike Alec Baldwin, I will say a prayer for him and I hope you will too. Only God can get your soul through the agony of knowing that you killed someone. It’s hell on earth to wrestle that level of guilt.