the question has arisen, here as elsewhere. i guess the first thing to say is that every killing technique kills people and also wounds and injures people; that's what all these things are for. so i'd sort of go piecemeal, and it would be silly to say that chemical weapons are the worst sort of weapons in every respect. but, chemical weapons are, first, entirely indiscrimate: they just kill or wound everyone in a given area, which is why they are grotesque when applied to, say, neighborhoods or inhabited regions. and they can't even be confined to the area you want to target even if you hit it, because they waft at the whim of the winds. and they are cheap and easy to make, so it is especially difficult to stop their proliferation. it's a task for the mullahs or the beloved leader to make a nuke and deliver it, or even a biological agent; i could probably whip up poisonous gas in my basement. of course the sorts of injuries and deaths inflicted by sarin are excruciating and horrific; you die slowly in one of the most painful ways possible. you can imagine a situation in which whole cities or regions could be gassed in a horrifying genocide; you turn a whole city into an instant auschwitz; saddam (with cia assistance) came close to this, but it could even be worse, and assad has the stuff to do it.
but if you're considering whether it would better to have your neighborhood gassed or napalmed, for example, you would face a difficult decision. if assad were napalming neighborhoods, i hope people would have the same reaction, and for god's sake i hope they'd be outraged and want to do something about it. the firebombing of dresden or tokyo, the nuking of nagasaki: right, these weren't any more wonderful or less indiscriminate than it would have been to use gas. now, cruise missiles actually are somewhat better in my opinion (look, we are moraly distinguishing ways of killing people, which always is going to look rather grotesque), for example, as is anything that on a good day can be reasonably precisely targeted so that you could try to limit their use to actual military targets. obviously, even in a best case you are liable to kill and injure innocent people. but still, there is a moral distinction between a weapon you can (if all goes well) drop right into the military command center, and a weapon whose nature forces you, if you use it at all, to take out the command center by annihilating everyone in its whole region.