Taking out the leader of a nation is a nontrivial step to take. In my view, it should only be done under extreme circumstances that would essentially meet the burden of proof beyone reasonable doubt.
Chemical weapons from different manufacturing sites can be differentiated from one another by analyzing their chemical makeup. We can tell, for example, that these chem weapons were manufactured in Syria, and the only ones with the ability to manufacture them is the Syrian government.
The problem is that the rebels have captured several weapons depots and have possession of some Syrian government chemical weapons. Therefore, it is impossible to know who used them in this instance.
Further, the goal in Syria should be stabilization, not regime change. There are quite a few reasons why, not the least of which is the risk of creating a power vacuum which radicals would be more than happy to fill. If stabilization occurs, THEN bring charges against Assad. If matters proceed in any other way, then the US does not win any friends on the world stage.