This was not an accidental discharge. This was a
negligent discharge.
Just because I feel like adding something...
It's popular around here to push the idea of AD's being mechanical failures and Nds being rules violations.
Negligence has a very specific meaning. Negligence is a deriliction of reasonable standards of conduct which results in harm to others.
As such, a mechanical failure induced discharge can be a negligent discharge if, as an example, the manufacturer didn't design or QA the components in conformance to industry standards. Likewise a human-induced discharge -- including one that causes bodily harm -- can be an accidental discharge if, again as an example, the human was attempting to follow standard conduct but failed in ways that any reasonable person might fail.
It doesn't do us ANY favors to accept an expanded definition of negligence. The only thing it does is give civil attorneys ammunition to attack gun owners.
The primary division is reasonable conduct. If in general your conduct was based on reasonable caution you were not negligent. In this case the guy was cleaning a firearm, thought he had cleared it, and did the necessary step of relieving the striker by pulling the trigger. The gun was aimed in a way that was unlikely to harm others. Not negligence. The second issue is consequence to others. In this case nobody else was harmed so it was not negligence.
Falling off a bicycle isn't negligence. If you make a gun that a reasonable person would know is going to be unsafe to fire and hurt yourself, that's not negligence. If you sell the gun to someone and they hurt themselves (or someone else) it becomes negligence.
If you handle a gun in a way that a reasonable person would know is unsafe, but you do it in a private area where others are very unlikely to be hurt, that is not negligence. If you do the same thing in a crowd and hurt someone else it becomes negligence.
If you handle a gun in a way that all reasonable people would consider to be safe (e.g. you are carrying a drop-tested pistol in a holster and trip) and it discharges it may not be negligence even if someone else is killed.
AD means "nobody else was harmed (in body or property) OR the harm was not preventable by standard conduct of reasonable people."
ND means "gun discharged and BECAUSE of a recognized violation of standard conduct of reasonable people someone OTHER THAN the violator of standard conduct came to harm."
This was an accidental discharge. The person made a mistake, hurt himself, and the bullet stopped short of harming anyone else. The only way it could be called negligent is if you are arguing that Glock was negligent in designing a firearm that needed a trigger actuation to disassemble, or maybe that didn't have a loaded chamber indicator.