BEIRUT, Lebanon — After dragging 46 bodies from the streets near his hometown on the Syrian coast, Omar lost count. For four days, he said, he could not eat, remembering the burned body of a baby just a few months old; a fetus ripped from a woman’s belly; a friend lying dead, his dog still standing guard.

Omar survived what residents, antigovernment activists and human rights monitors are calling one of the darkest recent episodes in the Syrian war, a massacre in government-held Tartus Province that has inflamed sectarian divisions, revealed new depths of depravity and made the prospect of stitching the country back together appear increasingly difficult.

That mass killing this month was one in a series of recent sectarian-tinged attacks that Syrians on both sides have seized on to demonize each other. Government and rebel fighters have filmed themselves committing atrocities for the world to see.

Footage routinely shows pro-government fighters beating, killing and mutilating Sunni rebel detainees, forcing them to refer to President Bashar al-Assad as God. One rebel commander recently filmed himself cutting out an organ of a dead pro-government fighter, biting it and promising the same fate to Alawites, members of Mr. Assad’s Shiite Muslim sect.