Saaliha Khan scrolled through her Facebook feed and pored over posts expressing sadness, dismay and disapproval, the kind of messages that always appear after an attack carried out in the name of Islam. It was just past midnight on Monday, and two Muslim gunmen had been killed about six hours earlier in a shootout outside a controversial cartoon contest in Garland, Texas, which featured images of the Islamic prophet Muhammad. “If you want to defend the honor of the Prophet, do so in a way that is consistent with prophetic manners and ethics, not violence,” Omid Safi, director Duke University’s...