Due to the fact that they whole purpose was to protect non-Human Chaos Worshippes, they love of (Chaos) Animals and they regular fighting against Chaos Warbands that believe in Human supremacy and Human survival and dominance via Chaos, the Indigo League has good chance of developing somehow misanthropic behaviors and general disgust and hatred towards Humanity.
For example:
++++Inquisitorial Report++++
DATA-RUNE REQUIRED FOR DOCUMENT ACCESS.
ANALYSIS...
DATA-RUNE ACCEPTED.
This document collect the general informations, concerning particularly vile breed of Traitors. For they not only sold they souls in service to the Ruinous Powers but also betrayed the Human race, putting even lowliest Xeno above the Mankind.
The Indigo League are a insane Warband of Fallen Astartes, painting their armaments in vile and garish hues, and who's seemingly formed solely for guarding a minor Chaos corrupted Xenos species, know variously among both Ordo Malleus and Ordo Xenos as the Scalings, The Servitors of Pain, Saalatiun or The Serpents of Regret.
This Xeno-breed is according to the few written records concerning the Xeno collected by Ordo Malleus operatives and ramblings of interrogated cultists to have, in it's foolishness, sold their souls to one of the Dark Powers for either survival or some other faulty reasoning, long lost to bygone ages.
Other characteristics of this Warband is their almost childlike adoration for Fauna, especially that tainted by the Immaterium, which they often use during their raids. And of course the strangest, their seeming cooperation with each other and brotherhood akin to their Pure counterparts in the myriad Chapters across the Imperium, a thing unheard of among the damned servants of the Warp.
This Warband due to their position as protectors of at least one Xeno-breed, often wages war against their fellow slaves of the Ruinous Powers, for many of the Fallen Astartes, even depite their corruption, still cling to ideas of Human domination across the stars and view the actions of the Indigo League with disgust and contempt. Unfortunately for Mankind, every such conflict didn't end up with mutual destruction of the Warbands, nor the extinction of the League and their Xeno-pets but in overhelming victory of the League time and again.
However, what's this vile Warband, is best know for, are their nightmarish raids on worlds of the Imperium. Woe is the tale of the unfortunate "survivors" of such raids.
Any Human will be round up like livestock and transported to the world of Curnto, stolen by the Indigo League through fell sorcery. The twisted Astartes are know for abusing the kidnapped baseline Humans, beyond even the horrors of other damned servants of Chaos. Making out of Humans living decorations, furniture and garments to decorate their halls and temples to their Dark Patron, rest on and parade in. The abused victims, partially fused and partially sewn together, are forcefully keep alive using the unholy tech-sorcery of the Servitors of Pain, and also constantly pumped with hallucinogenic drugs to cause as much confusion and fear in the Warbands victims as possible, the screams and weeping of the Human amenities and clothing, relaxing cantata to the Fallen Marines.
But the primary function of the Humanity to the Warbands is that of livestock not material, breed in separate regions of their world for different purposes. Some of the populations are breed for new recruits, the process of "adoption" and "purification" of acolytes saturated with ritual, both cultural and arcane, to create hard divide between the Indigo League newly born "little brothers" and the lowly livestock from which their "siblings" are derived.
Others are breed for obedience and hardiness, used as work animals, the counterpart of standard slaves in other Warbands.
Most populations however are breed for taste, their only destiny that of sustenance for countless mutated monstrosities belonging to the Warband. Those populations are also most often harvested to be rearranged into Warband amenities.
The Indigo League is also known for doing raids not for new livestock or material, Human or otherwise, but for sport. What was once productive planet, sending Tithes of Guardsmen or minerals, now, nothing more than wasteland of gore the mutilated and half-eaten corpses filling the ruined cities to the brim, for the Warband is know for organizing "battle tournaments" between their favorite beasts, and in those, Humans are used as target practice, or snacks and playthings given to the individual Marine beast, after a victorious "turn" by their monstrosity.
Note by Saarios Faceel, Inquisitor of the Ordo Xenos: The Indigo League is know for actively avoiding committing raids on worlds inhabited by Xenos, and even, there were noted examples of the Fallen Astartes of this Warband, doing unholy pilgrimages to Xeno civilizations for the purpose of spreading the corruption of their Dark Patron.
I didn't use Sessetium actual name nor "Slaanesh", because I wanted to show that this is in-universe document by a very specific and biased people, and thus said people, despite their vast resources, just don't know everything by virtue of not having access to wikipedia articles.