Here's the one I struggle with. So they can be ripped apart by 7.62mm rounds, eviscerated by random 81mm mortar rounds or vaporized by air dropped munitions but somehow using CW crosses a line?
Agreed - the argument that some forms of lethal weapon are less morally acceptable than others is difficult to understand when looked at dispassionately.
Some might argue that WMD do not discriminate between "civilian" and "military" targets - but that distinction has become blurred in recent asymmetric conflicts (and the other side are not always to blame) . Some might argue that WMD are strategic weapons in that they enable an attacker to inflict mass casualties with minimum risk to their own personnel - but we already do that by using drones and cruise missiles where the bearers of deadly power bear no personal risk.
I've never read a rational argument for the illegality of WMD compared to so-called "conventional" weapons which are just as indiscriminate and deadly, and which require no risk on behalf of the military personnel who deliver them.
It is not really even clear that CW should be classified as WMD. They are by no means as capable of "mass destruction" as nuclear or advanced biological weapons. At worst, in purely destructive terms, they are no more powerful than many legal so-called conventional tactical weapons.
Ironically, their only real strategic value is that they are "classified" as WMD. So as we saw in the 2003 in Iraq, their nebulous existence was "justification" to start a war that continues to cause horrific casualties to this day, and as we see today in Syria it is possible for elements of the international community to be duped into alarming attacks on entirely spurious evidence of their misuse.