Still trying to figure out how it happened.
The slide was locked back at one point. When he dropped the slide, there must have been a loaded mag in the pistol (or possibly a loose round in the chamber, which makes very little sense, but possible), and no one noticed that there was ammo in the gun?
Not to mention that if he'd treated every gun as if it was loaded, kept it pointed in a safe direction, or kept his finger off the trigger . . . any ONE of those safety rules would have prevented this. He had to break at least three of the four basic rules in order to shoot himself.
You try to make things foolproof and the world keeps breeding better fools.