World War One's chemical weapons usage led to such brutal results that the vast majority of countries vowed never to repeat Germany's example. Other than Saddam's gassing of the Kurds and/or rebel Iraqi factions, wouldn't that make Syria the only nation to use chems against its opponents?
What about Great Britain, Spain, Italy, and Japan, for starters? Though perhaps not themselves a nation at the time, what about the Russian Bolsheviks?
In addition, what about Agent Orange?
Or, are you arbitrarily restricting the list to nations that explicitly targeted people?