I don't want to seem to be making light of the dreadful attack in the suburbs of Damascus, which seems to have killed hundreds, but why are some method acceptable(high explosives, for example) while gas or chemicals is not?
I can't see how long it takes a target to die being much of a concern to the military. That it's banned by the Geneva Convention is probably the reason why it's attracting such flak.
Good question. I think it partly relates to the perception that they target civilians (and combatants) in a particularly nasty and widespread fashion. However many of the 'conventional' weapons attacks have been just as indiscriminate and deadly. And of course other countries have used other types of devices such as thermobaric and cluster bombs in wars
you are correct, the military may not be concerned about how quickly the victims die but the whole point of your question is about acceptability. Bio/chem weapons are generally seen as nasty slow painful deaths. So, as far as you can have "rules" in war, they are more frowned upon that convetional bullets and explosives.
Once the rule has been put in place and agreed by all then it is seen as objectionable - in fact a war crime - to break it. It then becomes a question of use it if you think you can get away with it