The way i see it is that the West is knocking over the regimes that have a history of not towing the party line.
In essence they are trying to create a Middle East totally under the control of the USA and Europe.
Russia and China know this is part of the great game of power politics and are therefore understandably trying to block what is in essence a land grab by the West.
Syria is just part of the overall battle plan. The main prize is the control of Iran with its vast Oil and Gas reserves, as well as vital trade routes between East and West.
In addition Iran borders the former Soviet republic of Turkmenistan which has massive reserves of natural Gas and Oil. This Gas and oil is exported to Russia, China and Iran. In part because in the past the Russians have blocked them from selling gas and oil to the west.
If Iran is run by the USA then Turkmenistan merely has to build a pipeline into Iran and out to the Persian Gulf to make all the west happy. Alternatively they could run the pipeline via Iran, Iraq and Syria into the Med. (I'm sure you can spot the pattern developing here.)
By taking control of the Iranian and Turkmenistan oil and Gas the West would then be able to control China and Russia. No play ball, no gas and oil.
A future Iran being run as a puppet state of the USA, would also provide an excellent location for the placement of strategic military resources.
These would allow the West to exert military influence on failed states like Afghanistan and Pakistan.
Emerging powers like India and China would also note that the USA would now have bases that allow their nations to be directly threatened by tactical, and not just strategic aircraft.
China in particular would probably feel boxed in by the growing American military influence via bases in Taiwan, Japan, Korea, and Iran
Control of Iran by the USA would give the Americans their desired FULL SPECTRUM DOMINANCE.