OK, now the theories for why Syria spiraled so wildly. The first is what you might call "sectarian re-balancing" or "the Fareed Zakaria case" for why Syria is imploding (he didn't invent this argument but is a major proponent). Syria has artificial borders that were created by European colonial powers, forcing together an amalgam of diverse religious and ethnic groups. Those powers also tended to promote a minority and rule through it, worsening preexisting sectarian tensions.
Zakaria's argument is that what we're seeing in Syria is in some ways the inevitable re-balancing of power along ethnic and religious lines. He compares it to the sectarian bloodbath in Iraq after the United States toppled Saddam Hussein, after which a long-oppressed majority retook power from, and violently punished, the former minority rulers. Most Syrians are Sunni Arabs, but the country is run by members of a minority sect known as Alawites (they're ethnic Arab but follow a smaller branch of Islam). The Alawite government rules through a repressive dictatorship and gives Alawites special privileges, which makes some Sunnis and other groups hate Alawites in general, which in turn makes Alawites fear that they'll be slaughtered en masse if Assad loses the war. (There are other minorities as well, such as ethnic Kurds and Christian Arabs; too much to cover in one explainer.) Also, lots of Syrian communities are already organized into ethnic or religious enclaves, which means that community militias are also sectarian militias. That would explain why so much of the killing in Syria has developed along sectarian lines. It would also suggest that there's not much anyone can do to end the killing because, in Zakaria's view, this is a painful but unstoppable process of re-balancing power.