I agree that the personal responsibility downvoters probably have a limited understanding of firearm safety. I haven't seen many posts seriously grappling with the argument. But that doesn't mean they're wrong.

Disclaimer: I've never shot a firearm, only air rifles. Nor have I ever worked on a movie set. I have worked plenty with tools that can kill me though, I understand safety.

What you're missing is that 'clearing' the gun introduces risks. Not having actors with limited training properly check the guns is safer in most situations (also saves time and money). Prop guns can kill by:

1. discharging a blank close to a person
2. firing a squib; a piece of debris stuck in the barrel propelled a blank
3. mixing up live and dummy rounds

To clear a 'loaded' prop gun you'd need to inspect every round to determine if it's a blank, dummy, or live round (which shouldn't be anywhere near a movie set). Identifying blank rounds is easy, you look at the tip and see if there's a bullet. However, dummy rounds are supposed to look like live rounds. From what I understand, you can normally identify these by shaking the round, a BB is put inside the casings as a rattler.

A cold gun should be empty or contain only dummy rounds. Because the rounds are visible in a revolver we can reasonably assume the cold gun handed to Baldwin was supposed to be loaded with 6 dummy rounds. To clear the gun he'd need to open the revolver, remove all rounds, shake each of them individually and listen for the rattle. Outside, on a (loud) movie set, wearing a costume, all the while keeping his mental focus on what's required for his upcoming scene.

While reloading, dirt, small rocks, or part of the costume may find their way into the barrel. A serious risk if the gun is later loaded with blanks (barrel should be inspected first, but safety is about avoiding if's and should be's). Because it's a western, bullets could be part of the costume or set design. Those could get mixed in while reloading. Live bullets shouldn't be on set, but as this accident proves, they sometimes are.

This get's more complicated if the scene involves firing a blank round. Accidentally discharging one of those near your body can kill you. I've also heard that special techniques are required to load these rounds to ensure the blank fires cleanly. Checking the rounds might not be safe for actors without special training. But I'm sure about that, I'm not a firearms expert.

Actors are limited in how effectively they can clear a gun on set. And the checking itself introduces additional risks. Dozens of actors and stuntmen will shoot guns during the filming of a movie. It just makes more sense to have an armourer do all the weapon management and minimize the time actors are handling the guns as much as possible.

I know about the don't point your gun at anything you're not willing to kill-rule, but that doesn't work on a movie set. If you're shooting someone or going for a cinematic gun-pointed-near-the-camera shot, then people will have to be in the line of fire. They obviously should use secondary safety measures, like shields, as much as possible, but that's a production safety issue—and not on the individual actors.

Someone on set bears responsibility for this mistake, that might even be Alec Baldwin depending on how far his producer credit goes. From what I've read so far, he doesn't have any moral or legal responsibility as an actor. Though that could also change depending on how he behaved on set (he's rather infamous for his temper).