I'm sorry, look at it from the totally legal issue. Put aside your personal feeling of what you think of him first.... this is a movie set, he is just the average dufus actor who has no clue as to firearm safety and on top of that the whole point of the movie is to point guns at people and pretend to shoot them... somebody (the expert on the set) handed this guy who is clueless about guns one that is supposed to practice shooting people with. Whose fault is it that there was a live round in it? This is not you or me but just the average clueless American. I bet half the clueless people out there if you handed them a revolver that you said was unloaded would start pulling the trigger and not have a clue as to how to check it.
I say it ain't his fault, he is an idiot but that does not make him guilty.