Charging document here:

https://s3.documentcloud.org/documents/23593041/baldwin-statement-of-probable-cause.pdf

Basically, Baldwin is being charged because, as an actor:

He only attended 1/3 of his required firearms training session, and didn't pay attention during the part he was there for;

Did not complete the industry standard safety check with the armorer or assistant director to ensure the gun was unloaded;

Knowingly used a prop gun capable of firing live rounds during a rehearsal when he knew that industry standards required the use of a plastic gun;

Had his finger inside the trigger guard while manipulating the gun;

Pointed the gun at the victims which you should never do (the victims should not have been standing where they were, but given that they were standing there, Baldwin should not have pointed the gun at them);

Pulled the trigger while the gun was pointed at someone which you should never do. Baldwin contests this last point, but the FBI tested the gun and found it incapable of spontaneously firing as Baldwin claims.

As a producer:

Generally failed to enforce industry safety standards regarding handling of firearms and also actively encouraged certain unsafe behaviors. Specifically in addition to the above, the armorer should not have been assigned additional duties that took her attention away from gun safety, the director and cinematographer should have been monitoring the camera feed remotely, not standing in a spot where a gun was pointed at them. Also some other things.

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Basically, he was in charge of the set and let and encouraged a bunch of unsafe behaviors. Also, as the guy actually handling the gun, he had a responsibility to do so safely even if nobody else was doing their job to ensure safety. Even if the guy who gave Baldwin the gun told him it was "cold," there are other things Baldwin should have done or refrained from doing to ensure nobody got hurt.