In all actuality I'm not blaming Alec, I blame the armorer who gave him a gun without checking it and secondly - why was live ammo on a movie set? Part of the script was for him to shoot at the camera (where a bad guy would be standing). It was not up to Baldwin to check the gun for a live round. Why would he?
PS: I'm not defending him, just stating obvious things a defense lawyer would bring out.