One concern is that it's now cheaper, easier, and faster to mass produce weapons which will evade metal detectors. I don't think any of this should be illegal, but the concerns should be considered.
It strikes me as likely that a 3D-printed gun will not be the most effective way to accomplish almost any kind of harm for almost any kind of perpetrator, no matter how situated. And that gives me a little comfort, even if I wish my countrymen were less fascinated with weapons.
But they also won't pass metal detectors.