A good starting point for understanding the Syrian civil war is Sunni-Shiite hostility. Before the civil war started, Syria was analogous to Saddam's Iraq in reverse. Both were ruled brutally by the minority sect. But in Syria it is the Shiites (the Alawite religion is a branch of Shiism) that rule over the majority Sunnis.
The west and the Sunni Arab world would like to depose Assad so that Syria could be democratic or ruled by Sunnis. There are certainly plenty of Sunnis in Syria opposed to Assad, in secret where Assad has control. But the educated ones who prefer liberal democracy are unlikely to try to overthrow him. And the Sunni extremists who are willing have a different agenda. They want a Sunni homeland and many have allied with disaffected Sunnis in Iraq under the Islamic State banner. They are interested in attacking Assad, but are perhaps more concerned with setting up their own separate country.
This is a four way conflict. Brutal mostly-Shiite Loyalists, brutal Islamic State, brutal Al Qaida (Sunni but enemies of IS), and a western backed opposition (maybe less brutal). The first two parties are big gorillas, and the last two are largely symbolic. We have chosen to back a mouse in a fight between gorillas.
Enter Russia. They (and of course Shiite Iran) are backing one of the Gorillas. This is a brilliant way to increase their influence and a wonderful opportunity to humiliate the west. It is no surprise they will attack the opposition, since their sole objective is to overthrow Assad. Thus the danger: the west and Russia are now militarily invested in opposite sides.
This presents a conundrum. Do we continue to arm the opposition and risk a proxy war with Russia where we are backing a mouse against their gorilla? Do we withdraw and maybe Assad can stabilize a broken Syria or even push out IS with Russian help, giving a huge victory for Putin and Iran? Or do we retaliate by arming Ukraine and crushing the Russian insurgents there?
Very dangerous indeed.
