The Syrian rebels posed casually, standing over their prisoners with firearms pointed down at the shirtless and terrified men.

The prisoners, seven in all, were captured Syrian soldiers. Five were trussed, their backs marked with red welts. They kept their faces pressed to the dirt as the rebels’ commander recited a bitter revolutionary verse.

“For fifty years, they are companions to corruption,” he said. “We swear to the Lord of the Throne, that this is our oath: We will take revenge.”

The moment the poem ended, the commander, known as “the Uncle,” fired a bullet into the back of the first prisoner’s head. His gunmen followed suit, promptly killing all the men at their feet.