Comment by Spivak

What you're saying is true but needs context. Chemical weapons aren't very effective in war because you need high concentrations spread over large areas, the wind is your enemy, full body clothing is common and and gas masks are cheap.

But if your target is an unsuspecting small population in an enclosed space who's spending a lot time there the calculus changes a bit. Sarin for example is odorless and colorless, mustard gas can also be colorless, doesn't hit you immediately and unlikely to be detected by smell.

It actually happened in Iran and it's lucky the people responsible either didn't know what they were doing or were actively trying to not kill people because they easily could have.