It seems that I've read that not all 'rebels' are equal.   At least at first, there were home grown Syrians who hated Syria's oppresive authoritarian goverment.   Supporting them was clearly a controversial item in the Obama administration.    After supporting a democratic election in Egypt then abandoning the election process for fear of encouraging a budding Muslim theocracy, I suspect the Administration wasn't anxious to open a full fledge war front.

Isis is more an outsider group capitalizing on the now damaged and weak Central goverment as on opportunity for them to gain power.

It seems what you are supporting, and please correct me, is that we should have done what Russia has done and back Assad?   

Hmm, it wouldn't be the first authoritarian goverment the US has supported, we have many examples around the world, with Iran and the Shah being the most well know in that region   (Some would say Israel is in that group - when it comes to their Muslim population).