Here is an excellent piece in the NY Review of Books on the armed group ISIS operating in Syria.
The most interesting question to me is who is behind this group. All accounts point to foreign fighters as the core and their opposition to other Syrian rebels is well known.
For me there are three choices:
This is Qaeda from Iraq: Syria and Iraq share a border and the group from Iraq is expanding westward into Syria in furtherance of the restoration of the Caliphate.
This is an organization financed by the Gulf states using imported fighters from a diverse Islamic regional base who transit through Turkey in order to support Sunnism.
Assad is behind this group. Creating increased savage violence in rebel held areas (in the wake of the loss of their chemical weapons stockpile) forces Syrian citizens to grudgingly accept the Assad regime.
And while I’m certainly no expert in Syrian history, my bet is for option 3.