You can turn a microwave into a weapon that bursts people's eardrums, intestines, stomach, causes an intense burning sensation under the skin, as well as permanent heartband brain damage.
You can turn a bunch of blue ray writers into a laser cannon that burns holes through people.
Don't get me started on how easy it is to make IEDs or chloroform, or even just how a mass stabbings make no noise and people don't realise whats happening until they're covered in blood, unlike a shooting where you instantly know what's happening and from what direction.
3d printed guns are the least of my worries, anyone who can access ammunition for the guns could break down the ammunition to make a detonator for some plastic explosive which is just petroleum jelly and potassium chloride.