Himself, while cleaning gun
From the article I will assume it was a Glock, now we can't blame the booger hook on this one, Glock's do have to be de-cocked to take apart. How about not following instructions? All the Glock manuals I've seen show to grasp the take down tabs from beneath the firearm, but how many people do you see grasp the tabs from the top?
Which leaves some part of the hand in front of what? Muzzle you say? You are correct!
What did we learn in basic firearms class?
Always point the gun in a safe direction
That muzzle is always pointed at something