The truth is there are some very nasty people among those opposed to Assad, and by no means all of them split away when many did to be a central part of Isis when this was set up. To assume that backing the rebels in 2013 (while those who set up Isis were still part of the phalanx of "moderate" opposition groups) would have ended with a democratic regime replacing Assad is extremely optimistic.   

At all times it appears that the Islamist fighters, whether in Isis or not, are perhaps the most effective, and it does not seem likely that they would have been ready to take a back seat in any new dispensation, especially one attempting to operate in secular or non-sectarian way.

It's a particularly horrible conflict, as civil wars almost invariably are. How far Assad is any different in his behaviour than other heads of State in such conflicts is unclear. Lincoln was accused of slaughtering "his own people", and it was true enough. Nor is it by any means clear whether those opposing him were more representative of the people of Syria or less. Many things have happened that pretty clearly were and are war crimes. But similar things and even worse can be said of what Americans did in Vietnam. And none of their leaders ever stood in the dock, though we may wish they had.