The Doctor finds himself in an "unholy alliance" with Sec, realizing that a Dalek with a soul is a preferable alternative to a galaxy of genocide. By intentionally interfering with the gamma radiation strike atop the Empire State Building, the Doctor infuses the hybrid army with Time Lord DNA, granting them the freedom of will they were initially denied.
This thematic tension is mirrored in the setting of the Great Depression. In the "Hoovervilles" of New York, humans are at their most vulnerable, yet they retain their identity and empathy. The Daleks, conversely, are at their most powerful technologically but are spiritually bankrupt, unable to see the strength in the "imperfections" of humanity. IV. The Doctor’s Intervention [S3E5] Evolution of the Daleks
The remaining members of the Cult—Caan, Jast, and Thay—represent the traditional Dalek dogma of "purity". Their eventual mutiny against Sec highlights the self-destructive nature of Dalek philosophy. To these Daleks, any deviation from their original form is a "sickness," even if that deviation is the only path to avoiding extinction. The Doctor finds himself in an "unholy alliance"
The central conflict arises when Dalek Sec, the leader of the Cult, merges with the human Mr. Diagoras to become a human-Dalek hybrid. Unlike his predecessors, Sec begins to experience "humanity"—feeling regret, compassion, and an appreciation for the "great survivors" of the universe: mankind. In the "Hoovervilles" of New York, humans are
Sec’s realization is profound: the Daleks are failing because their obsession with universal supremacy is a strategic dead end. By seeking the Doctor’s help to relocate his new race to a peaceful planet, Sec attempts to redefine what it means to be a Dalek, trading extermination for evolution. III. The Ideology of Purity vs. Progress
The tragic conclusion—where the traditional Daleks slaughter their own hybrid offspring—reaffirms the Doctor’s long-standing view: the Daleks’ greatest enemy is not the Doctor, but their own refusal to change. V. Conclusion