Negligent discharges (ND) are not only dangerous to people who are around when the bullet exits the muzzle, but they are also dangerous to all gun owners. A single negligent discharge may run on the evening news or on the front page of the local paper, whereas the millions of people who safely handle firearms every day are never mentioned. A single mistake tarnishes all of us.
In this case, we have a commercial airline pilot who appears to have negligently discharged a firearm while the plane is on a cross-country flight. We can assume it was a negligent discharge because there is no mention of any reason the firearm would have been intentionally discharged.