“Prop guns” include rubber dummies for background use like in war movies and real guns that are deadly if there’s real ammo in them instead of blanks. And even blanks can kill under the right circumstances. It’s also possible a blank sent some sort of debris out the barrel. It’s happened before.
The real question would have to be, why was live ammunition on the set? I'd suspect old hot-head Baldwin had a score to settle with those two people and hopes nobody can prove it wasn't an accident.
I know that guys in Ranger School will take a deer with a section of cleaning rod using their issued blanks.