I can inject you with a biotoxin, killing you and no one else.
This is also not -necessarily- the case.Also, any incompetence of the builder while making a gun at home cannot lead to massive death tolls of those in the surrounding area.
I can build several thousand simple guns,all with the same flaw, killing several thousand people.
My argument against NBC weapons has been noted. I dont think you need to go past that argument to effectively argue against their "casual" posessionIf there existed some form of discriminatory bioagent that could affect the intended target and intended target only, then I would be in favor of that weapon becoming legally possesable by the public at large. But such a bio-weapon does not exist.