From a geopolitical standpoint, 1500 people dying from chemical weapons in Syria is an appetizer to greater brooding conflicts, all of which directly surround the resources in the middle east and Africa. There's no such thing as morality when everyone has their hands in a resource-cookie-jar built around explicitly murdering, manipulating or exploiting entire countries for the sake of stealing or controlling resources.
Thus, no one can very well step in to intervene on the moral high ground against this injustice when the very precedent for the conflict and the framework for continuing to acquire said resources globally is implicitly nefarious and depends on constant warfare and exponential military growth to function. That's the market. And the G20 controls the market.
Yes, chemical weapons are a red line that shouldn't be crossed. But because it's a red line drawn overtop so much spilled blood that it's not fully seen for the violation of humanity that it is--instead, it's more like a continuation of our decent. I mean honestly, we're in the third world war already, but nobody wants to admit that it's about resources and market shares rather than principles and humanity.