You're right that 'it's not weapons that kill people, it's people that kill people'.....
However, you might want to question what you read more.....you can't, thankfully, 3D print an AR15 that will successfully operate for more than a couple of shots.....3D printing as most people know it is something done in sheds using poor quality plastic and requires software and other skills to even look neat. 3D in industry is a marginal technique that demands extensive knowledge, equipement and space.
Additionally, it is fortunate that auto weapons require a supply of consistent ammunition to operate and even those capable of making viable firearms can't usually make the ammo in their sheds.....fortunately!
The whole 'plastic gun' scare is larfgely the product of media......but has been the source of fools trying to duplicate. So, not just "anybody" has all the requirements and facilities to make a machine gun and ammo....fortunately!