Both sides in the Syrian civil war are bad guys. Both sides may possess chemical weapons. If one side uses those weapons, it seems to me that the other side would be inclined to retaliate in kind. It doesn't even matter who started it.
If killing a bunch of people with chemical weapons is really no different from using conventional weapons, then I suppose it is not a big deal if this war becomes an exercise in turning neighborhoods into open-air gas chambers, setting the precedent for future wars to look like that.
But, if there is a difference....
Let's go ahead and assume that the Syrian government is recent events. If Bashar al-Assad has used chemical weapons before, why wouldn't he do it again, in the absence of pressure to stop? If these Syrian militants include al-Qaeda fighters, what makes you think they would avoid the temptation to use chemical weapons, whether supplied by Saudi Arabia or taken from captured Syrian stockpiles?
And if it is some sort of 11-dimensional chess false flag operation, well, if I were Assad, I might want to go down swinging and do the things I've been falsely accused of doing because I am being blamed for it anyways.
The longer this action goes without a response, the more likely it is that someone will use chemical weapons again. So, there is some undefined time constraint if the rest of the world doesn't want to see this civil war descend into tit-for-tat poison gas attacks.
The question then becomes, what sort of pressure is necessary to convince both sides to come to heel? Does Assad need to feel that any further use of chemical weapons may cost him the war? Do the rebels need to feel that Assad has been appropriately punished by the international community? Are both sides a bunch of barbarians who only understand a fist or is there a diplomatic answer?
If it is not worth trying to bring an end to the war directly, is it an acceptable goal to discourage the further use of chemical weapons or retaliation in kind, so that both sides can go ahead and kill each other so long as they use conventional weapons? If so, what is the minimum effort necessary to increase the chances that no more chemical weapons are used and is that effort non-military in nature?