Hassan Aboud, left, with Abu Ayman Muslim Terrorists

Behind the Black Flag: The Recruitment of an ISIS Killer

Hassan Aboud’s practiced baritone belied the malevolence in his words.

“Oh Darraji!” he sang. “Our state provided us ammunition and sent us to assassinate you.”

That state is the self-proclaimed Islamic State, also known as ISIS or ISIL, the terrorist group that controls territory in Syria and Iraq and has recently projected violence to Ankara, Beirut, Paris and San Bernardino, Calif. A soft-spoken double-amputee sometimes carried to meetings by fellow gunmen, Mr. Aboud is an Islamic State commander who also directs a network of assassins, including those who killed Darraji, a former subordinate, with bullets and flame.