My wife works in props for the theatre and has regaled me with all the rules and regulations that go along with prop firearms. While working at a range in California that would occasionally host a studio shoot I had the opportunity to pick up a lot of the rules. I don't really have an opinion on what happened other than it is terrible for everyone involved. Just a little insight to add to that huge article a couple posts up.

There's one final rule if you're firing a prop gun regardless of the distance. You don't point it directly at someone. If all the other safety protocols fail you have that last step when the trigger is pulled. A decent cinematographer/camera operator will set up a shot so you can't tell. If you've ever watched a movie or show and clearly sees the actor pointing the gun to the side it's what they're supposed to be doing. It just wasn't captured well. I would have thought Alec has had enough experience to know that. As I understand the shot was him pointing the gun at the camera hence the cinematographer losing her life.