Neither of the belligerents in the conflict in Syria are "good guys". Some of the rebels are the same fighters who fueled the insurgency in Iraq. al Qaeda publicly called for the overthrow of Bashar al Assad.
Assad is a bad guy, but removing him does not install a better government. If Assad falls, the chances of chemical weapons falling into the hands of al Qaeda are high. Ethnic cleansing (against the Shiite Alawites, Christians, and Armenians) is likely under any kind of government led by the Sunni rebels.
If the Assad regime falls, it will destabilize Iraq, put us in a proxy war with Iran, and further endanger Israel. We tried nation-building in Iraq and Afghanistan, and look where we are. We cannot fix Syria. Have the courage to stay out; stop arms shipments to the rebels.