Assad is fighting a rebellion, not engaging in deliberate massacres. Killing 10,000 civilians (the commonly-accepted death toll) can be accomplished in a matter of days. The Japanese killed hundreds of thousands of Chinese during the Nanking Massacre over a period of weeks. Hafez Assad killed 10,000 Syrians over the course of a week in Hama, back in the 80's. 10,000 dead over 16 months is actually lower than the rate at which Iraqi civilians were being killed during our occupation of Iraq. (That rate peaked at around 30K a year in 2006 & 2007). In other words, it's about what you'd expect for a guerrilla war.
Besides, what do we care if Sunnis are getting slaughtered? These troglodytes are our enemies and have been the enemies of Christendom for 1300 years. Their neighbors would not miss them if they were teleported to another galaxy. Would Thailand, India, the Philippines and China miss their Sunni terrorists? At the risk of sounding calloused, maybe Sunni should move to a more welcoming locale, like Saudi Arabia, whence their ancestors emerged to spread terror and death around the globe.
Syria is simply not worth going to war with a united Russia and China. The US will lose that war badly. Quite frankly, the US leadership during this Syrian debacle has been one of the worse examples of diplomacy in US and perhaps world diplomatic history. Total disregard for the safe keeping of thousands of chemical and biological WMD’s.