Most chemical weapons break down rapidly, yes. That is why those chemicals were used as battlefield weapons - for an army, rending the field of battle impassible for any length of time is a major bug, not a feature. There are known toxins that are far more persistent, and they could be deployed as WMD far more horrific than very large explosions. None of them have been used, or even proposed as weapons, but this speaks of the restraint of weapon makers, generals and politicians, not the space of what the laws of nature make possible.
I would prefer not to give examples because there is a diffrence between people learning of this while studying in general and providing searchable hits for "How to end the wirld", but go find a list of poisons by toxicity, and read up on their specific properties. There are /many/ things worse than nukes. Either we are in a timeline surrounded by an void of death or humans can in fact be trusted with the keys of Armageddon.
In order to kill someone with a poison, the poison has to at least touch that person, and many poisons need much more than skin contact to be lethal: they have to be ingested, injected, or inhaled. Explosions tend to expand rapidly. Chemicals, however, tend to sit in one place; you need a way to disperse it. Yes, a handful of botulinium toxin could theoretically kill every human on the planet, but you'd never be able to get it inside every person on the planet.
In other words, [citation needed].
On the other hand, I wouldn't be surprised if one could kill more people with a few vials of smallpox virus than with a single nuclear weapon...
The standard view of Mutually Assured Distruction (MAD) is something like:
Occasionally people will reply with an argument like:
This is an anthropic argument, an attempt to handle the bias that comes from a link between outcomes and the number of people who can observe them. Imagine we were trying to figure out whether flipping "heads" was more likely than flipping "tails", but there was a coin demon that killed everyone if "tails" came up. Either we would see "heads" flipped, or we would see nothing at all. We're not able to sample from the "tails: everyone-dies" worlds. Even if the demon responds to tails by killing everyone only 40% of the time, we're still going to over-sample the happy-heads outcome.
Applying the anthropic principle here, however, requires that a failure of MAD really would have killed everyone. While it would have killed billions, and made major parts of the world uninhabitable, still many people would have survived. [1] How much would we have rebuilt? What would be the population now? If the cold war had gone hot and the US and USSR had fallen into wiping each other out, what would 2013 be like? Roughly, we're oversampling the no-nukes outcome by the ratio of our current population to the population there would have been in a yes-nukes outcome, and the less lopsided that ratio is the more evidence that MAD did work after all.
[1] For this wikipedia cites: The global health effects of nuclear war (1982), Long-term worldwide effects of multiple nuclear-weapons detonations (1975). Some looking online also turns up an Accelerating Future blog post. I haven't read them thoroughly, and I don't know much about the research here.
I also posted this on my blog