Historically the opposition to chemical weapons has a lot to do with how ineffective they were at actually killing compared to leaving victims suffering instead.
The ban on CW that came after WW1 was partly driven by the fact there were lots of survivors with horrific injuries in the post-war society. In 1920, 2 years after the war ended, something like 60,000 died as a result of injuries received from CW during wartime. That prompted a specific movement against use of the weapons that were causing such harm long after the war had ended.
But I agree, it does still seem like an odd distinction to make at times.
Don’t nukes do the same thing? Of course, if it was outlawed the leftists and western countries would give up theirs and believe all the other countries did too.