I think my question is, can't we be outraged at both the killing of civilians and then also the fact that chemical weapons were used?
We should be outraged at the indescriminate killing of civilians, The method by which they are being killed should not determine our moral decision on whether it deserves our attention or not.
Is the use of Sarin really morally different from hacking them up with a machete, or burning them in their homes, or hanging them in the street, or bombing them with conventional ordnance?
A war crime is a war crime and how we determine our response should not be dependent upon the type of weapon used to murder or the objectives or motivations of the faction employing such weapons.