JMA,

the legal situation is far from clear as the action of Assad's forces would not have been against another state actor but own population.

Actions against own population are from a more legal point of view very complex. They are according to an expert in this field not allowed, however, only could be punished under current legislation with UN mandate, which is of course in this case blocked by Russia and China.

http://www.swp-berlin.org/fileadmin/...013A54_slr.pdf

(sorry, article is in German)

So the ugly conclusion of the author is that without Russian and Chinese cooperation there is no firm legal ground to support punitive actions against Assad.

BTW, before such action I would prefer hard evidence and a clear statement what "we" would do if rebels were the culprits, a case that would open another can of legal worms.