I want to call out something, since I feel a connection is being drawn here that is not valid. His gun was not 3D printed.
Investigative sources said that the gun used in the incident consisted of two metal cylinders wrapped in vinyl tape, which could fire six projectiles when the trigger was pulled. When prefectural police examined the seized weapon, they found it was equipped with an electrical cord and battery and that it was designed to ignite the gunpowder with an electrical current.
I'm not taking issue with most of what you said, but FDM 3D printers aren't the issue here. Making a gun can be done at home with essentially no tools. I don't think we should get pulled in to blaming societal issues on a single piece of technology.
I'm in a hurry so I've left this comment short, I may come back and edit it with more thoughts later.