Mustard gas is ineffective. That is the actual reason it’s outlawed: The opposing force dons gas masks, completely negating the effect, the only stuff that it still kills is collateral damage. That’s precisely what happened during WWI: It made everything nastier without actually having an impact on the strategic level.

There’s this notion among many people that the Geneva convention is about preventing cruelty or something, not at all: It’s about preventing pointless cruelty. Cruelty that does not actually serve a military objective. War is hell, that’s already a given.