Sure if you can sleep at night with all that blood on your hands. When the NRA even agrees that a gun product should be outlawed it's pretty bad.
I'm an enthusiast, and the first thing that went through my mind when I saw those a few years back, was that's the next thing to be banned. That said, for all the time they've been available, this is the first and only time they have been used for something like this. It's also important to note that they aren't necessary to make a weapon behave like this...it just facilitates it.
Cool to see more folks playing with 3D printers. I'm guessing you guys are starting to see now why very few make large 3D printed parts to sell. Unless they are very rare and often sought after items, the design time, print time, maintenance and materials usually makes it not worth the effort, or the final cost of the item too high to be reasonably marketable. But they are great for tinkerers, making personal "one-offs" and prototyping for other manufacturing processes. Not sure we'll ever see machines based on this type of technology which would be well-suited to replace the more standard methods of production.