All the reports suggesting that the Syrian government may be preparing Sarin for use against its own people are alarming, The fact that the US and the UN appear to deem the reports credible means the danger should be taken seriously. I am generally cautious about reports coming out of Syria; there are too many competing factions, producing propaganda of their own, and too many videos of uncertain provenance being posted to social media with little ability to verify the source. The recent AP "Iranian nuclear plan" fiasco is a reminder that false information (in that case, leaking a graph that did not show scientifically what it claimed to) can easily be reported around the without proper verification.
What the US has said so far is that there is evidence Syria is bringing together the chemicals needed to manufacture Sarin, which is a binary weapon. There is no doubt that Syria possesses a sophisticated binary chemical weapons capability; this is not a case of an alleged capability like WMD in Iraq in 2003. The fact that the US has been so adverse to open intervention up to now suggests the alarm has some justification, though that does not mean that some caution is not in order.
Despite stern warnings from the international community, it is far from clear that this would deter the beleaguered regime once it decides it has no alternative but to use Sarin against its own people. That would mean the regime is prepared to bring the building crashing down on its own head: that the Alawite community is prepared to go down fighting even if it carves a space for itself in the annals of genocide.
But if the threat is real. one hopes some thought is being given to how (other than verbal warnings) to deter the actual use of such weapons in this already horrific war.