Not everyone's idea of democracy is our idea of democracy. And what rebel groups can you determine are good. And how is a group of rebels arming against their government democracy either. Assad apparently enjoys significant support in Syria, because many Syrians are extremely worried that Assad going means a Fundamentalist Sunni government. Under Assad's regime, women had equal rights and everyone had full citizenship under Syrian law no matter what your religion was. That's not exactly the "democratic goals of the rebels". And can we control exactly what rebel group ends up in power. The most powerful one are the sectarian ones.
Look I'm not saying lets support Assad. He's a bad man. But I am saying supporting armed rebellions isn't the answer too, and I have no doubt we have fueled that fire. Reports are a lot of those "rebels' are really foreign fighters.
Personally my feeling is I don't support funding dictators or groups that abuse people and minorities. And you know what that feeling transcends to a lot of groups in the Middle East including a lot of our allies. Our Assad is bad but Saudi Arabia is perfectly fine only shows hypocrisy to the folks in the MIddle East.
And Assad isn't perfect but he's kept a lid on a lot of things.