I know next to nothing about guns. If one bullet was fired, how is it that one person was killed and another wounded? Does that mean the same bullet struck both of them?
Pardon my ignorance.
Remember these guns are not loaded with bullets. They are referred to as "live round", but the charge is usually a quarter the normal gunpowder load. Then the bullet is capped with either with cotton or paper wadding. Now remember the last related major tragedy is the time in 1984 actor Jon-Erik Hexum placed a blank load against his head and the impact of the fill killed him.
Now we have to guess until the information comes out. I'm guessing the director was standing next to the cinematographer lining or shooting a take. Baldwin must have been instructed to shoot at, or toward, the camera. Since the bullet shell does not have a solid object (a bullet) but instead wadding spewing out with great force, perhaps parts of it went in more than one direction (but close by.)
The crew was partly composed of last minute non-union crew members. As I said the regular crew members had expressed concerns about on site safety issues. One complaint is they weren't getting their morning safety briefings. This is unheard of with professional crews when "live rounds" on used on the set.