The US has used chemical weapons, specifically in Vietnam. And I tend to agree with your point on international law, as alluded to in the parenthesis of my response. Although at least one person here ignores the moral argument, focusing his argument entirely on the date which the prohibition of chemical weapons was enacted. However, if you look at this from a moral perspective, then you must consider the use of weapons that have not been banned, such as the use of depleted Uranium shells, which have resulted in many birth deformities in Iraq.
1) On killing people, the US is an expert on that.
2) On using chemical weapons, the US can count itself a member of that club.
3) On using unguided bombs, Syria does not have the luxury of modern precision guided bombs.
4) On shooting down civilian airliners, the US can also count itself a member of that club.
Specifically on (3), which is commonly presented in western media as proof of the Syrian government's targeting of civilians, it reminds me of the criticism of Algeria in their war of independence (anther incredibly barbaric war for the sole purpose of preserving French colonialist control). The then commander Ahmed Ben Bella was asked why he was hiding bombs in pushchairs to target the French, he responded that (and I am paraphrasing): If they will give us some of their helicopters and aeroplanes, then we will give them some of our pushchairs.