I am always suspicious of claims that some "enemy" has engaged in "outrageous" acts with no apparent rational justification. Can anyone explain a motive for Assad to authorize a single limited poison gas attack on a specific location that included hundreds of children? What benefit could he possibly accrue from this action, which would invite military retaliation from the US?
The first thing you have to do when trying to figure out the rationale of doing something like this is try to remember that the person making the decsion may not be as rational or as moral as you are.
There are two scenarios where this was a deliberate act by the syrian government to get rid of some pesky areas of resistance.
In the first case its from the top, an Assad decided to end the problem, figuring that he's been blasting his own people with conventional weapons for almost a year now, and the world response has been tepid. He probably expected what we are getting now, some half assed attempt at protest and possible action, but looking less likely.
The 2nd case is some local high level commander got uppity and decided to fix the problem with whatever he had availible, and he happened to have a few sarin shells lying around. In this case one would expect said local commander to be either praised or killed, depending on how the Assads see what happened.