Here's a thought: is it really "illegal" for there to be a civil war within a country? Think about that: no few nations have been born, or changed, for better and for worse thanks to a war within their own borders, both with and without international participation.
Under what auspices were the various protagonists in the Bosnian conflict brought to book? There are UN conventions on human rights etc relating to not deliberately targeting civilians, genocide etc. I haven't read the legal justification yet but perhaps it relates to the fact the Syrian leadership has knowingly broken these.
Cameron has a point - if we do nothing now, we in effect legitimize the use of CW. Tricky situation, as I can't imagine what we could possibly achieve through a limited strike, other than either maybe persuading Assad to be more careful or re-double his efforts using conventional forces.
The stakes are also getting very high with many players angling behind the scenes - Israel must be gearing up save they get drawn in (they could use our intervention as a final justification, if ever one was needed to go after Iran).