The woman from Sealand justifies the eradication of the Waknuk people by asserting that a conflict must always come between a species that has advanced powers and the early form of the same species that has lesser powers. John Wyndham suggests, through the voice of the woman from Sealand, that a certain incapacity of mind--a certain incapacity for an expanded logic of mind--makes accepting advanced evolved human traits virtually impossible for humanity.
Wyndham carries this to its logical conclusion and has the Sealand woman declare that the higher form of the species must annihilate the lesser form. Wyndham's real hope is that readers will come to understand how lives are confined by the logic of the present order of life--a logic that cannot adapt to catastrophic reshaping of life and the world--and then learn to think "outside the box," as we might say today.
The woman from Sealand justifies the killing of the Waknuk citizens by stating that the Waknuks are no longer a superior form of the species.
Her explanation is very Darwinian. Adaptations lead to various differences in overall fitness levels of organisms within a species. Individuals with high fitness are more likely to survive and advance the species. Individuals with low fitness are naturally selected to die out. The woman from Sealand doesn't have a problem with killing the Waknuk people, because the woman from Sealand views the Waknuks as the group that nature is naturally selecting to die out.
"For ours is a superior variant, and we are only just beginning. . . The essential quality of life is living; the essential quality of living is change; change is evolution: and we are part of it."
The woman from Sealand also has a much simpler reason for killing the Waknuk people. She states flat out to David and the others that the Waknuks were going to kill David. In order to save David and his friends, the Waknuk pursuers had to die.
"They are alert, corporately aware of danger to their species. They can see quite well that if it is to survive they have not only to preserve it from deterioration, but they must protect it from the even more serious threat of the superior variant."