I don't know much about weapons of war, so I can't be confident of the details here, but consider this thought.
Suppose an army has a choice between two kinds of weapons. The two are equally lethal, but one kills quickly while the other leads to a slow, painful death. That seems to be a good moral reason for using the first rather than the second. The enemy soldiers will be just as dead, to use your phrase, but the world will have been spared some suffering. The chemical weapons that those World War I poets wrote about—mustard gas, for example—were so horrifying precisely because they killed so slowly and so painfully.
I take it that's the reason (or at least one reason) for treating chemical weapons differently from bombs and guns. And on the face of it, it seems like a pretty good one.