In late June, images made their way around the world of four men as they were locked in a car and killed with a rocket-propelled grenade. They showed seven men, chained together with explosive necklaces, as they were blown up. And they provided evidence that five men had been locked in a metal cage and lowered into the water to drown. As we learned last week, 16 men in total were murdered in these brutal ways. We know this because the executioners with the group calling itself “Islamic State” wanted to film their victims as they were dying.