Even then it makes little sense... 3d printers are just tools. They can be used to print dangerous items, or parts of dangerous items in the same way a saw or hammer could be used to make something dangerous. To some degree this is just a problem with human nature – some people are going to want to harm people and will create or acquire items which do that.

Perhaps if it was literally as simple as downloading a model and pressing print, then in 20 minutes you had a fully working automatic rifle this would be an issue, but that technology simply doesn't exist today.

In reality if your goal is to acquire a weapon which can do lethal harm to someone you just wouldn't print a gun. Even if you wanted to kill multiple people in a place like the UK where guns are illegal you still wouldn't print a gun because you'd probably be better off just getting knife than printing a crappy gun and trying to source an effective propellant, etc.