Once a Legion dedicated to perfection in all its pursuits, the Emperor's Children succumbed to the call of forbidden knowledge, making them victims for Slaanesh.
In the Black Library novel Fulgrim, author Graham McNeil explains the evolution of the legion as being one of seeking perfection, to art, to pleasure, to experience, to hedonism. When I read this novel I was somewhat unconvinced that this was a logical progression.
McNeil provides an interesting lens through which one can partially view the transition: that of first captain Julius Kaesoron. Kaesoron accompanied his Primarch Fulgrim during the Cleansing of Laeran and was present during the attack on that xenos race's capital city as the genetically-modified serpentine Laer warriors died to protect their central temple which was dedicated to Slaanesh. Like many of those that entered the temple, Julius was enraptured by the sights and strange discordant music as the cacophonous flood of sensations assaulted him with a surge of light and noise in the building.
Thereafter, Julius obtained the writings of Cornelius Blayke from the expedition's archivist. A visionary from childhood, Blayke had, it appeared, been affilicted with visions of an ideal world where every dream and desire could be realized. This Cornelius Blayke is almost certainly a nod by McNeil to the real life William Blake, an English poet, painter, and printmaker. Largely unrecognised during his lifetime, Blake is now considered a seminal figure in the history of the poetry and visual arts of the Romantic Movement. The Romantic movement was a 19th century response to the 18th century age of Enlightenment, which had posited similar to the 30th millennium Imperial Truth that man could stand unaided at the center of his own rational universe. Blake was notable for his belief that there was no separation between spirit and matter, a rejection of authoritarianism, and a belief that sexual activity should not be criminalized.
Perfection is a term that is hard to define. To some being perfect means being complete. To others, being perfect means something exactly correct for its purpose. Going with the latter definition, what is the purpose of mankind? One could argue that, absent any assurance of eternal truth, our purpose may be to seek to embody the the purest and most complete form of positive experience in material form. This positive experience can be expressed in the form of modes such as art, beauty and pleasure. A quest for perfection / pleasure nevertheless often relies on certain rather questionable assumptions: such as (1) that perfection is unchanging, (2) that perfection is achievable in the material world, (3) that ultimate pleasure is in fact a worthwhile goal and not a subversion of life's purpose, and (4) that pleasure is valuable even if it provides no concrete gains based in reality.
The Emperor's Children seem to equate pleasure with excess sensation. If pleasure is good, more pleasure is even better. Continued exposure to sensation runs the risk of becoming jaded, requiring more extreme exposure. Once again, there are a number of questionable assumptions made in this progression: (1) that pleasure can remain perfect even if it causes negative outcomes to others, such as pain and harm of others, (2) that pleasure can remain perfect even if it causes negative outcomes to oneself, such as underdevelopment, addiction, long term loss, lack of empathy or failure to evolve, (3) that excess sensation (possibly including pain) is in fact more pleasurable, and (4) that something can be perfect even if it is ephemeral and fleeting in nature, requiring greater and more extreme future applications.
Here we reach the central issue. The Warhammer universe is grimdark in nature. There is no perfection or eternal salvation in this setting: only a brief spark of reason which will inevitably be snuffed out in time. A quest to achieve perfection in the material world therefore is bound to be futile, because there is no eternal truth to the universe (whether physical or spiritual) other than chaos. So the Emperor's Children quest for perfection was always doomed to fail.
McNeil further expounds upon this harsh reality in his novel Vengeful Spirit. In Chapter 17, Raeven of the Imperial Knight House Devine meets Fulgrim, in the guise of the White Naga. As he approaches the demon, the landscape erupts with sensation. Fulgrim tells him that there is "no such thing as too much", and introduces himself as the "ontological ideal of perfection." Raeven resists, insisting that there is "no such thing in the world as perfection" and that if a thing were perfect "it could never improve and so would lack true perfection". "Perfection depends on incompleteness!" he admonishes the serpent, prior to striking it and running for his life.