When you have two sides, both devoted to the worst kinds of evil, it's not wise to pick either one. Indeed, from a moral standpoint, neither the Syrian government nor the rebels can be supported.
While it's true that we supported the Soviets in WWII vs. the National Socialists, there was a direct benefit to Britain and the U.S. by supporting the enemies of the Germans.
In this case, no one can benefit from either Syria or rebels winning. We shouldn't get involved unless there is a major third faction that is not devoted to totalitarianism and islam and it's extremely unlikely that such will appear in that country.