Anons, a word on aircraft safety.
This anon worked in the airline industry for quite awhile. The first thing you learn, the greatest fear, is of a plane going down. This is not an advertisement; rather, it's common sense: when an airplane goes down, airlines go down with them. See "Valuejet."
I believe that this is Southwest Airline's first fatality resulting from an aircraft incident. This is pretty impressive, when you consider how large it is, and that it has been in operation for 50 years.
Security was pretty good when I left (quite awhile ago). The job required a background check, which became much more rigorous after 9/11.
Inspections are routine and carried out by the mechanics, the pilots, and the ground crew to a certain extent. Southwest Airlines employees have an added incentive (as do JetBlue and certain other airline employees), in that they receive bonuses that may come in the form of company stock.
Difficult to speculate on what might have occurred without giving people bad ideas, but the point is that, as Q noted, it's highly irregular. I am less familiar with Delta's safety record, but I cannot remember anything noteworthy. They are an even larger airline, with longer flights.