Mens Rea.
It's that simple.
If Wil Wheaton had pointed the gun at Kiefer Sutherland in Stand By Me, and shot Sutherland, of course Wheaton would be innocent. Wheaton was a child at the time. He wouldn't be expected to know how guns work, or how to check the gun. He might have not even know whether or not it was a real gun. Hell, I still don't even know.
Baldwin knew, by his own words, that he was operating a real gun. Baldwin also, by his own words, knows how dangerous guns are. Baldwin also, by his own words, opted to not check the gun himself while being able to do so.
For the dozenth time, I own a better version of the same gun that was on Baldwin's hand.
If I'm operating that firearm, and somebody ends up dead by that firearm, I go to prison.
I go to prison because of mens rea.
I go to prison, because I know that my gun is real. I know that my gun is a deadly weapon. I know how to check my gun.
If I point my gun at a human being, and the gun goes off, I'm culpable -- even if somebody handed me the gun, and told me that it was empty.