LONDON — The images have become as familiar as they are repellent: a victim, swathed in orange, kneeling, forced to recite a propagandist’s script; a black-clad, masked executioner rearing over him, knife in hand, declaiming his own lines from a litany of anti-Western grievances before seeming to perform ritualized murder in front of a video camera in the desert.

The intended symbolism is unmistakable in these excerpts from the uneven war against the Sunni militants of the Islamic State: The victims, emblems as much as they are citizens of Western lands, must kneel before a newer power. The grisly act of decapitation represents the inexorable nature of that power.

With the executions first of the Americans James Foley, reported on Aug. 19, and Steven J. Sotloff, on Sept. 2, then of the Briton David Cawthorne Haines on Sept. 13, the militants acted out a parable of their own dominance.

In Britain, there was another dark strand to this deadly narrative. In all three beheadings, the hooded figure speaks with what seems to be a British accent. A British militant, in other words, is depicted as ending the life of a British aid worker equally far from the land that molded both the jihadist dream of one and the philanthropic vision of the other.