The special repugnance we have for death by WMD is sort of like "hate crime" statutes. Hack'em to death machetes? Not a good thing, obviously, but boys will be boys, don'tcha know. Treat them to a horrible form of insecticide? Unacceptable, and someone will send a harshly worded letter to the editor.
The major reason we are all up in the trees about WMDs in general, and nerve gas in particular, is that we fear such an assault by terrorists on our soil, and so want to be able to credibly threaten nuclear retaliation while maintaining our smug and sanctimonious sense of moral superiority. I fully embrace the notion of American exceptionalism, btw, but the particular weapon we choose to smite evil has nothing to do with it.