Purity

The Thirteenth Doctor observed that the Daleks were "pretty obsessed" with purity, ironically for a species born out of mutation. (TV: Revolution of the Daleks)

The Dalek Emperor in the Last Great Time War insisted that his Dalek of human origin were not "half-human", an observation from Rose Tyler which he found blasphemous. He claimed that everything human had been purged from them, that he had "cultivated pure and blessed Dalek." (TV: The Parting of the Ways)