Bodies littered the road out of El Geneina, a town in western Sudan, as Dr. Rodwan Mustafa and his family sped down a bumpy road that led to the border with Chad and, they hoped, safety.

A day earlier, rampaging Arab militiamen had grabbed Dr. Mustafa by the neck, accusing him of giving medical care to enemy fighters. That was his signal to run.

Racing toward the border with his family in a car, he saw chickens clucking over the bloodied corpses of those who hadn’t fled in time. A camp for displaced people stood empty, burned to the ground. He spotted a dismembered hand on the roadside.

“The smell of death was everywhere,” said Dr. Mustafa, who made it to a refugee camp in Chad and spoke by phone from there.