There are a few reasons, both historical and legal that matter, so bear with me for this mini-essay.
Simply put, the use of chemical weapons violates the law of war. More specifically, the use of chemical weapons violates the Chemical Weapons Convention of 1993. Conventional weapons are given more leeway (legally speaking) in conflicts under international law (See The Convention on Certain Conventional Weapons
It’s not really so much about their efficiency, rather it’s about how brutal they are. Blister weapons, nerve gases, and their cousins are not pleasant ways to die, they aren’t usually containable (meaning that once you fire them, the environment determines how far they spread, who they effect) and finally chemical weapons can have a long term environmental impact on areas – causing long term injuries and death after the conflict has ended. Think of them like invisible landmines, where even in decades post-conflict they can still maim and kill unintended targets.
Historically, of course, examples of chemical weapon attacks only reinforce the brutality of the weapons. World War 1, the Halabja Poison Gas Attack during the 1988 Iran-Iraq War.
But I also wanted to point out something about the way you phrased the question. Mass murder during conflict with any sort of weapon is generally considered fairly heinous (i.e. War Crime). The killing of soldiers or a large amount of soldiers during a conflict is not legally ‘mass murder’ (moral questions aside). So it’s tricky because mass murder is a crime, either way, you cut it, generally speaking, the mass slaughter of civilians by any sort of weapon sparks outrage and in the worst cases intervention (except Rwanda… Darfur…. most of Kosovo etc). For more information check out The Fourth Geneva Convention on the Protection of Civilians
The current situation in Syria (presuming that’s what sparked the interest) is not just about the use of chemical weapons (though it’s certainly a major part) – it’s that the Syrian government deployed the weapons against its own civilians. That’s a massive problem in international law for the reasons I’ve listed above
Steven Pinker talks about this in “The Better Angels of Our Nature” particularly in the section “IS THE LONG PEACE A NUCLEAR PEACE?”. Here are some relevant excerpts.
It’s not immediately obvious why, out of all the weapons of war, poison gas was singled out as uniquely abominable—as so uncivilized that even the Nazis kept it off the battlefield. (They clearly had no compunction about using it elsewhere.) It’s highly unpleasant to be gassed, but then it’s just as unpleasant to be perforated or shredded by pieces of metal. As far as numbers are concerned, gas is far less lethal than bullets and bombs. In World War I fewer than 1 percent of the men who were injured by poison gas died from their injuries, and these fatalities added up to less than 1 percent of the war’s death toll.
One possibility is that the human mind finds something distinctively repugnant about poison. Whatever suspension of the normal rules of decency allows warriors to do their thing, it seems to license only the sudden and directed application of force against an adversary who has the potential todo the same. Even pacifists may enjoy war movies or video games in which people get shot, stabbed, or blown up, but no one seems to get pleasure from watching a greenish cloud descend on a battlefield and slowly turn men into corpses. The poisoner has long been reviled as a uniquely foul and perfidious killer. Poison is the method of the sorcerer rather than the warrior; of the woman (with her terrifying control of kitchen and medicine chest) rather than the man.
Whatever abhorrence of poisoning we might have inherited from our evolutionary or cultural past, it needed a boost from historical contingency to become entrenched as a taboo on the conduct of war. Price conjectures that the critical nonevent was that in World War I, poison gas was never deliberately used against civilians. At least in that application, no taboo-shattering precedent had been set, and the widespread horror in the 1930s about the prospect that gas-dispensing airplanes could annihilate entire cities rallied people into categorically opposing all uses of the weapons.
What do you think?