I think it's a mix of things on why he was charged.
He was a producer of the film, and as such he was responsible for things such as safety. So it's like if a company's actions hurt somebody, the person in charge can get in trouble.
He did fire the gun, he did not check to see if it was loaded. I wonder if it's really reasonable to check something like this every single time. Imagine how many scenes and takes it takes to make a movie. Take after take, scene after scene. Imagine how many times he's fired a gun on a movie set.
And lastly, Alec Baldwin, while one of my favorite personal actors, has earned a reputation for being very liberal and anti-gun. So he kinda painted a big target on his back, despite earning a paycheck doing the things he's protesting against. So maybe there is some kind of politics at play. I really hope not. This should be about Justice for the person that died, not some political vendetta.
My viewpoint on this is that every gun on every single film set in the USA should be made to where it absolutely cannot be used to fire a real bullet. This has happened multiple times now, with Brandon Lee dying in the early 90's filming "The Crow".
IMO, all firearms, whether real or non-functioning, should be kept under lock and key, with a chain of custody from the armorer all the way thru to the actors. My guess is that Hollywood has gotten lazy with regards to making sure these guns aren't real and aren't loaded.
One of the first things you're taught as a gunowner is NEVER aim a gun at anybody you don't intend to shoot. And if you do aim a gun at somebody, be prepared to kill them (don't take it lightly). At least, that's what I was taught. You always act as if the weapon is loaded, even if you KNOW it's not.
I understand that you have to aim it on somebody on set to film a movie, but maybe there should always be some kind of bullet proof barrier between the actors and the film makers.