Leaving aside the other intelligent species in the galaxy, over the countless millennia since humanity first arose on Terra and spread across the galaxy, there had been countless humans and cults who worshipped Khorne, the Chaos God of the Warp, dating back to the distant age of ignorance.
For example, there were the broadly defined cults of Khorne, which revered violence, anger, and cruelty, and hoped to wield shadow and magic; the Crimson Skull Order, which encouraged people to hate and seek revenge, to commit violence, to resist and fight, and which eagerly sought to destroy human governments and spread disorder and chaos by killing and building towers of bloodied skulls; and the Sisterhood of Brass, which lured women into battle, promiscuity, slaughter, and hatred, until they ultimately fell completely into Chaos—and many, many others.
In short!
There were countless cults that worshipped Khorne, the Chaos God. The examples above were merely some of the more notable ones.
Besides them, there were also many more obscure cults with faith in Chaos!
They had infiltrated every corner of human society throughout the countless millennia of humanity's development. As the ages passed, and their cults succeeded or were destroyed, their members fell to Chaos one after another and continued to dwell within the terrifying Warp, favored by the beings they called their "gods"!
And now, Canopy Star, a planet on the verge of being consumed by the Warp, was naturally under the control of all kinds of Daemons pouring out of the Warp and Cultists who worshipped Khorne but belonged to different cults. They slaughtered and occupied the world.
Among them, the followers of the Crimson Skull Order—a secretive, shadowy cult that had existed on the planet since the days of planetary government—had successfully infiltrated every branch of the government's armed forces