The problem with printed firearms is that they're plastic. We have no means to detect them. They instantly obsolete our security infrastructure. You can walk onto an airplane with one. You could walk into a courtroom with one. You could walk into the White House, Congress, or the Supreme Court with one. That is a major problem.
And banning them will do exactly nothing to address that problem.
A person who would make a gun with the intention of committing murder with it isn't likely to be deterred by a law banning his gun. Actually, that law already exists... the Defense Distributed guy was careful to epoxy a six ounce block of metal to his before fully assembling it into an operable gun, because it's a federal felony to manufacture an undetectable gun.