Since everyone else is doing it, I figure I'll add my expert California housewife opinion...
I don't like the idea of a system that systematically disarms myself, my fellow passengers, and the crew and then selectively allows certain people to carry a weapon on an aircraft. How are the authorities to know for sure that this person is acceptable to carry a weapon. "Yeah, they're ok, until the gun goes off on approach to Charlotte...then they're not." Next it'll be at 36,000 feet except the gun was pointed back at first class? Sorry. I'm against it and it makes me feel less safe. But, it's the law of the land and I'll accept it as a necessary evil.
In my day-to-day life, I'll take it two ways: it's either everyone has guns or no one has guns for me. I'll accept either way, just make sure yours is bigger than mine if we go with the latter.
Obviously, this incident was a negligent discharge. What's disturbing to me is that the weapon in question and the angle at which it was fired, etc. looks like they had it out and were handling it in flight. Hell it was on descent. And by handling, I mean an object coming into contact with and then depressing the trigger. Given the objects in the cockpit that could conceivably press the trigger, the likely culprit would be the finger of the person handling the weapon.
So, in summary, someone picked up the gun, put their finger through it and squeezed back on the trigger. Given how most incidents of this nature occur, I'd say they were goofing around and thought it was unloaded when one guy thought it wise to "dry fire" the weapon and, surprise, a bullet shoots out...there was one in the chamber.
The moral of the story is never "think" a weapon is unloaded and never dry-fire a weapon or attempt to, regardless of if you "know" it's unloaded. It's just not wise.