Aloof. Polite. Cajoling. Extremely attentive. Restrained. Dishonest. Inscrutable. Malicious. The rebels from northern Syria, remembering encounters with him months later, recall completely different facets of the man. But they agree on one thing: "We never knew exactly who we were sitting across from." In fact, not even those who shot and killed him after a brief firefight in the town of Tal Rifaat on a January morning in 2014 knew the true identity of the tall man in his late fifties. They were unaware that they had killed the strategic head of the group calling itself "Islamic State" (IS). The...