come on now, that kind of thinking is why this technology took so long to take off.
to print that gun for use you need
3d printer $1000-$10000
ABS plastic - $45 per 1kg roll
take 230degrees to melt
bed temp needs to be 70-100degrees to properly stick (running the same power of an aircon/oven/stove)
you'll need between 15-40 hours of your life waiting for it
metal firing pin
you still need access to the bullets
then a clear mindset that you're going to use it.
you've already made clear that you're a fruitcake if you even kill someone with it, given the effort. I don't think the 3d printer made it easier, you made your mind up long before you hit print, which you could have just used your Australian built falcon and drive over someone instead.... those dangerous falcons
if you're a terrorist. you can sneak plastic through a detector, given that they don't xray your body or carry on. you've still got a metal bullet inside.
you've spent well beyond your needs for a one time use weapon
unless you're planning an attack in 2152 because you'll be waiting for parts to print its kind of silly.
on the subject of the liberator
its available on most pirating sites
that site has been taken down by the US
there is about 5 versions available now
you can print your own nerf gun
there's 5 guns to over 100,000 harmless (some useless) creations.
if anything the only problem 3d printers will cause is copyright infringements