such videos have also been used to sow terror and provoke an emotional overreaction that could enrage onlookers, expand a conflict or play into the attackers’ hands, said Amanda E. Rogers, a research fellow at the Century Foundation think tank who has studied extremist propaganda. Nearly a decade ago, she said, the Islamic State’s video-recorded beheadings of aid workers, journalists and others helped mark a turning point for terrorists who saw the value in publishing footage so heinous many viewers felt they couldn’t ignore it.