3d printers are not that useful they are crap.
Producing explosives is kind of science way beyond some retard level skill. And not that much use unless you want to do some mass destruction.
However there are some interesting and very cheap chemical weapons that are totally never used for any reason even if all components be found in the dumpster or purchased for relatively reasonable price. effectiveness should be something close to nerve agents except that slow acting.
probably it woud be even possible to genetically engineer some bio weapon at home but again way too much work and no real benefit. testing will be one of the biggest problems as you have no clue what it will do.
I have a concept of most powerful biological weapon as this woud be genetically engineered bacteria which produces drug like heroin. So when you get infected you will feel so good that you will not even bother to get cured LOL
That woud be an illness which kills you with pleasure. Maybe that could work for some guro story as well.