I know that Syrian history is full of war and atrocity- Crusades, Assyrians, etc. As a matter of demographics per capita, when was the last time that Syria experienced a level of warfare, death, and human displacement that is comparable to this? As a related question, was there ever a part of the French or Ottoman periods of dominion over Syria that was as bloody or as terrible as the current war?
Richard Bulliet, perhaps the most respected Middle East history authority, said, that from 1517 (when the Ottoman conquered Egypt) to 1830* (conquest of Algeria by France), the Arab World was effectively insulated from any violence or mass conflict. Compared to Europe which was in constant conflict throughout its history, until 1945 (and that is not even counting Eastern Europe/Balkans), the wave of violence that hit the Middle East post-1945, is unprecedented (with the major inter-war revolts being rather minor in comparison).
*And this is only really true for Algeria, we can push the date all the way to ~1883 for Egypt, or 1914/1915 for WW1.
That being said, the worst since the Syrian Civil War, was perhaps the Great Syrian Revolt against the French between 1925-1927. French control of Syria has constantly been under threat, as the French after removing "Greater Lebanon" from Syria, was never able to win the trust of the elite or common man. The rebellion saw the French using artillery, bombing and levelling suburbs of Damascus. When "elections" and "freedom" were introduced as a measure to calm the non-rebellious populace, the response was the revolt of Homs and Hama, whose notables (A'yan) did not trust French. The French ended up winning, but realizing the cost of suppressing the revolt, decided to cut back operations, much like the British in Iraq had done at the beginning of the decade. But still maintained it as a Mandate, until 1943 (when the Vichy French were evicted by British and Free French forces).
I'd also put up as another candidate, for "civil" insurrection, was Bashar al-Assad's father, Hafez al-Assad's, literal levelling of entire parts of the city of Hama in 1982, after the Muslim Brotherhood and other dissident elements, attempted to sieze power from him. The resulting massacre of the entire city, pushed the rebels, and specifically the Muslim Brotherhood underground until 2011.
Sources: The Arabs: A History - Eugene Rogan
Richard Bulliet's lecture series iTunes: "The Modern Middle East"
To be sure this is an [educated] opinion, no way of finding a true parallel, given the huge amount of destruction modern weapons can inflict.
EDIT: Please note that pre-modern conflicts, like the Crusades or the Assyrians, didn't really effect the common everyday man, as much as more modern conflicts, like the ones I listed above did. I don't think you could even compare the bloodshed to be honest, which was on an entirely different scale, with far more destructive weapons.