Until last year, Hassan Diab was leading the quiet life of a Canadian sociology professor. Prof. Diab was teaching at both Carleton University and the University of Ottawa, was said to be a popular colleague and teacher. After leaving the violence of his native Lebanon and earning his doctorate in the United States, Prof. Diab, 54, received his Canadian citizenship and appeared to settle into Ottawa. There, friends said he was a secular man with an interest in sociology and Middle East studies, and was not without a warm side. "He has a great rapport with students," said Carleton professor...