The problem is that generally chemical weapons are overrated.
The statistics you see for them regarding the incredibly small amounts of _____ required to kill one person are based on absolute perfect delivery of that drop; but all too often you'll see people simply divide some quantity of somthing by the minimum dose to kill and proclaim that that quantity of _____ can kill some bajillions of people - that's only if each drop was perfectly delivered under perfect, controlled, conditions.
In realtity chemicals get highly dispersed, degraded by sunlight, you get an overdose right near the attack and not enough at a distance, etc.
Chemical weapons were used in simply massive quantities in World War I and actually there weren't that many troops actually killed by them, relative to plain old high explosive - main effect was fear and the slowdown in operations caused by the masks, etc.
When Saddam gassed the Kurds it was a fairly massive, long-term attack in a pretty controlled setting; he probably could have killed them faster with shelling or just rounding them up and shooting them.
I welcome terrorists fooling around with chemical weapons (and the even more overrated "dirty bombs" as well) as any time and money spent on them isn't spent on high explosive attacks which would likely kill far more people.
If Aum Shinryko had put the time, effort, vast amounts of money, and highly trained people they used in the Sarin subway attacks into a simple high explosive attack like the London subway and bus bombings, they probably would have killed 10x or 100x more people than they did.
The main thing about chemicals, and the reason they're banned in warfare, is people are simply creeped out by them.
For whatever reason people are more freaked out by being paralyzed and dying from a drop of nerve gas than being painfully blown to pieces by high explosives and shrapnel.
The reason they're banned is because most of the casualties are crippled and the toxic debris that remains is a hazard.
"Chemical weapons were used in simply massive quantities in World War I and actually there weren't that many troops actually killed by them, relative to plain old high explosive - main effect was fear and the slowdown in operations caused by the masks, etc."
A great number of soldiers came home with damaged lungs. They were crippled for life.