I think Ns, Bs and Cs are all quite different.
Nukes are, at least in some ways, just like non-WMDs that already exist, but much bigger. So to a large extent I agree with Hansel’s analysis of the situation. If I were willing to drop 100,000 conventional bombs on a particular target, then I’m close to just as willing to drop a nuke, at least ethically (ignoring the issue of international censure).
Biological weapons are (to the extent that I know anything about them) not very useful on an actual battlefield, and incredibly hard to control. They’re good for inflicting mass casualties to a civilian population, or for depopulating the world… but I find it VERY hard to come up with a situation where I’d use them. And that’s not so much because they’re so uniquely evil (after all, dead is dead) but because it would require a hypothetical where biological weapons would actually accomplish something… maybe some crazy case where I live in an island nation, and an evil and heartless totalitarian state on the mainland has announced their intention of wiping my nation off the face of the earth and killing or enslaving every last one of its citizens, and my nation has zero chance of opposing them militarily, but has enough advance warning to release superFlu… but even if I could have a reasonable chance (since I live on an island) of preventing it from spreading to my countrymen, how do I keep it from spreading to the rest of the world, etc? That said, there was an episode of Battlestar Galactica in which our heroes, the only 40,000 humans left in the galaxy, were on the run from the pursuing cylons, who vastly outnumbered them and had already genocidally murdered 100 billion or so humans on 10 planets, and the humans happened on a way to potentially kill every cylon in existence, and one human took it on himself to screw up this plan because it was a biological weapon and that was cruel/inhumane. And I thought that was one of the stupidest things I ever saw.
Chemical weapons are different still. Again, they’re very hard to control, due to shifting winds and so forth. But I think a large part of the reason they’re so shunned is because of how painful and severe the suffering they inflict is. Combine that with the fact that lots of armies realized they just weren’t accomplishing much with them, and they’ve been banned since WWI. That said, if someone developed a painless and fairly-quickly-evaporating poison gas (not that I’m endorsing research into that topic!), I would have no compunction using it if national survival was at stake, as long as it was likely to actually accomplish something, as opposed to just killing for the sake of revenge.
War is hell… and if you’re already prepared to drop bombs that will rip people limbs off and eviscerate them and leave them begging for their mothers while they watch their own intestines fall out then it’s a bit weird to refuse to kill them with poison gas, even though that’s a status quo that we’re all used to.