|Roaches check in...But they don't check out|
But this is one of the arguments that sounds logical when you hear it, but kind of falls apart the more you think about it. Chemical weapons like Sarin and VX are essentially insecticides scaled up to kill humans instead of bugs. Why do we use insecticides? Because when we have a flea infestation, for example, it makes a lot more sense to use poison on them than it does to burn down the house. Even if there are risks and downsides to poisoning the fleas, the net outcome is a house still standing with no fleas in it.
Now think about a dictator like Bashar al-Assad or Saddam Hussein or Omar al-Bashir or any one of a dozen others. They have the infrastructure - pharmaceutical and insecticide production - to produce chemical weapons cheaply and in volume. They have dissident and/or insurgent populations within their borders. Usually, when they are faced with revolt or civil war they have to destroy a huge amount of their own infrastructure to eliminate the rebels. If they could just spray the towns and villages with human insecticide, wait 48 hours and march in and clean up the mess, these pesky revolutions would be easy to deal with.
Well, why don't they do that? Because ever since the end of World War One the world has had an ironclad convention against their use. Nations still stockpiled them - in the face of an existential defeat, one more bad decision isn't going to be a game changer. But it was clearly understood that to use them (without at least tacit agreement from the appropriate superpowers) was to risk significant punitive attacks from nations not even otherwise a party to the conflict. The idea was global deterrence, and it has worked pretty well. Of course, Saddam Hussein used Sarin gas regularly in the Iran-Iraq war and against Halabja in the al-Anfal campaign, but the US was allied with Saddam's Iraq in that war, and agreed to look the other way.
Now we may begin to see a breakdown in this generally agreed-upon convention. If it begins to be safe, even normalized for dictators to use chemical weapons against their own people in order to retain power, then we won't be seeing 100 dead here or a thousand dead there. We'll be seeing the wholesale slaughter of entire communities because it is faster, cheaper and more efficient than going in and fighting to take those communities back from the rebels. When we argue against an ironclad guaranteed imposition of a high asset cost for any leader who uses poison gas, we are merely helping create a world where the use of poison gas is commonplace.
And I promise you, dear reader, that is not a world you're going to want your kids to live in...