A common theme in 40k is also that physical purity often hides spiritual corruption. See the Black Dragons novel, Death of Antagonis, for an example, the moral of that tale is that there's a time and a place for purity of deed, word and form, what matters is purity of purpose above all else.
Many of the most virtuous beings in 40k look monstrous to modern standards, often scarred, genetically and mechanically enhanced and with lives extended long past their natural end. People like Saint Celestine are rare exceptions.
It also plays into the general theme that humans have had to sacrifice their own humanity to survive in the 41st millennium. In a galaxy riven by war, disease, apostasy and rebellion being human is not enough on it's own, and the things that allow survival make you inhuman. To refuse augmetics, genetic enhancement, super soldiers made from tortured children, cloned vat slaves, expending human lives more readily than light battle tanks, mass serfdom and the practice of turning the dead or criminal into cyborg servants is to accept being overrun by Orks, devoured by Tyranids, exploited by the Eldar or slowly picked apart by countless hostile xenos species.
EDIT: There's actually a question occasionally asked in various forms in 40k novels that I rather like, 'is this worth it?'
Asked in rhetorical fashion by a human who fought alongside the Iron Hands to retake her homeworld from a Slaaneshi cult in the final steps of turning the planet to a daemon world. She decided at the end of the story that if the Space Marines from the story were mankinds defenders, then humanity had long ago lost the war.
Also asked by the protagonist of Death of Antagonis as part of his gradual realization that various monstrous acts he's performed are the only way for the Imperium to survive, and that he'll have to do far worse in time and accept that far worse happens elsewhere and must be so.