Most plausible explanation:
* Propmaster and/or gun wrangler (it's a thing) don’t do their jobs thoroughly. They have some serious explaining to do as they are the ones that prep and load all firearms.
* Director wants a shot of Baldwin pointing the gun at the camera - similar to Goodfellas.
* Cinematographer is handling the camera, the director right behind her looking through the cameras external monitor.
* They call action, Baldwin shoots as directed.
Tragically, fatal prop gun shootings have happened before.
(son of Bruce Lee) was killed on the set of The Crow in 1993.