The issue about it being "negligent" is mostly whether someone wants to own up to it being they way they handle the gun.
Insert the mag, drop the slide is common technique. Whatever, the cause was the firing pin had enough force to impress the primer. Was the round previously loaded and the nose nicked, causing it to protrude? Too much lube on the firing pin, or no spring to keep it from bouncing forward? Plenty of things cause a discharge.
How it gets interpreted being "negligent" is a matter of opinion. If a defect is widely known, maybe. Handling the weapon during cleaning and having it go off is the most well known circumstance. In that light, a lot of people would call it negligent.
As for the hole, plain white toothpaste - a little dab will do ya. If it went through the roof, don't ignore it. Slip a piece of milk jug plastic under the shingle with some roof cement on it to seal it up.
I've seen negligent discharges (not me!) and I don't think you can exercise too much caution. It's the number one circumstance - loading and unloading. It's why I prefer rifles with detachable magazines - you don't cycle the ammo thru the chamber (less nicks) and close the cocked bolt on one just to extract it. But, you chamber a round by letting the action spring ram the cartridge home, semi autos all do that. Therefore, always point it where it won't be a problem.
If the pistol is kept after this, a bucket of sand in the room might not be a bad idea.
