If people could 3D print nuclear weapons that would be another story.
Don't worry that will never happen since one would still need Plutonium 239 or Uranium 235 and I bet it won't be available in form of filament anytime soon

I'm not afraid of 3D printed guns being used for common gunfights, anyone who really wants a gun can get one. The only thing that concerns me a bit regarding 3D printed guns is that they have better chance of passing through metal detectors especially if ammo is also printed from plastic. I suppose it would still be hard to get one on a plane but it could pass not so thorough inspections like the ones you can see on major events.

Anyways I'm no big expert on crime, I live in Slovenia where we have only 20-25 murders in the entire country per year. Population is a bit more than 2 million.