What makes chemical weapons so much worse than conventional weapons?
I wouldn't say that incendiary weapons such as Napalm "just kill" (Source 1). I can appreciate that differing levels of suffering are caused by different weaponry, it just seems that napalm, for example, is among the worst of the worst weapons. An even worse example (though this is arguably a chemical weapon, it isn't classified as such) would be another incendiary weapon; white phosphorous.
I think it's a difference between a quick death versus a long, painful, often psychologically damaging hope for death. Sure physical damage like a bullet, hurts and can have lasting effects if you survive, but chemical damage is just so much more and tends to stay in the body. You can repair a bullet wound better than you can damage from a chemical weapon.
That was an interesting observation so I looked it up.
"Incendiary agents such as napalm and phosphorus are not considered to be CW agents since they achieve their effect mainly through thermal energy. Certain types of smoke ammunition are not classed as a chemical weapon since the poisonous effect is not the reason for their use."