on why Russia and China vetoed a UN security resolution over Syria

Russia and China were never going to let pass a security resolution regarding Syria because of America's militarism, combined with their view that the Western European nations (read as: NATO) is merely a lap dog for the United States.  Russia and China have had bad blood between them for decades (if not centuries) yet both nations seem to have put all that past them as they seek to be a sort of international counter balance to perceived US aggression, especially now that it is quite clear that the drum beat for war is reaching Iraqian proportions when it comes to Iran here in the States.  Syria, having been mentioned as a possible foe if war broke out between the US and Iran, is, therefor, an important piece of the strategic puzzle to be neutralized by NATO or American forces. 

While I, and I'm sure, many folks around the globe are bitterly disappointed (the Syrian people truly do need some form of aid), one can't be surprised that Russia and China are naturally wary of yet another Western power military exercise in their spheres of influence and will do all they can to prevent it.