So from what I can surmise... it is not always a crime to kill someone. IF....
- Self defense.
- Not responsible.
And given that this was a movie set, people other than the actors have the responsibility to make it a safe and secure environment. Actors are given the tools / props to act with. In this case it was a real gun with real ammo. That is not the responsibility of the actor. Live ammo should not have been anywhere near them.
What makes this case, is that Alec Baldwin was also a producer. At least partially responsible for the production and safety standards that were apparently lax.
However... this is where it get real screwy. If the man, as the actor, was not responsible... then who was? The man who handed him a loaded gun, or the armorer who allowed a loaded gun onto set? Baldwin for hiring the armorer in the first place? If he even did. This is a game of hot potato as to avoid responsibility, each participant will assign blame onto the other.
So it comes down to specifics. Who did what, where, when, why?
In other words, it's a bloody mess and NO ONE here has all the facts of the case. None of us are fit to prejudge this.
If I was on a Jury for the case against Baldwin, I would need something MORE than him simply pulling the trigger. Did he order the assistant to grab a gun without the armorer present? Something like that. Otherwise I would let him walk as simply the man who pulled the trigger. I may not be so kind to the assistant or the armorer, though without them pulling the trigger... that might be a lesser charge.