A potentially in character argument for Chaos.
Chaos is often portrayed as the ultimate villain of the galaxy, the epitome of evil. In fact, all sorts of atrocities are committed with the justification that the terrible might possibly prevent some degree of Chaos incursion. Well, I would argue that Chaos isn't a villain at all and that it is the enemies of Chaos which are the true villains.
The Chaos gods are the true gods of mankind. It is the very nature of humanity which gives the Chaos gods their form. It is the courage, tenacity, ferocity and fighting prowess of humanity which empowers Khorne. It is mortal passion, our love of pleasure and desire for perfection which empowers Slaanesh. It is human intellect, curiosity, and drive to lear and explore which empowers Tzeentch. And it is our mortality itself, the briefness and hardship of each human life and the struggle against our inevitable demise which powers Nurgle. All of what is unique about humanity, all that makes us special, is personified in the gods of Chaos.
It is the Imperium and the followers of the God Emperor which suppress these natural elements of humanity in their fight against "Chaos." People's freedoms are tread upon, billions are killed as though their lives were worthless. People are not free to act or think as they desire. Even more insidiously, the true potential of human is suppressed. The destiny of human is to take their place along side of the gods as beings of the Warp, to become a powerful psychic race which can coexist with the Warp in perfect harmony. Yet the Emperor prevented this, it was his greatest fear that humanity might transcend its mortal frailty and become something greater. This is why for millennia countless psychics have been killed, why those who have been allowed to exist have been terribly persecuted and enslaved. These are the greatest among humanity, why are they so mistreated? Because of fear, because small minds fear humans becoming something greater than they are now.
Many of the problems of the Imperium stem from fear of Chaos, which is ultimately fear of the unknown or dogmatic adherence to policies created out of such fear. If humanity could coexist with the Warp, think of the ease of travel they might experience, think of the ease of communication. Imagine the strength humanity could wield on the battlefield if they embraced the powers of Khorne and allies with the Warp, which is essence part of themselves, instead of fighting against it. Think of the knowledge that could be gained if humanity opened itself to all learning and embraced the wisdom of Tzeentch, instead of clinging to past knowledge and rejecting new ways ideas.
If humanity embraced its true nature, embraced its true gods, then it could join those gods of the Warp and ascend into a new and greater race, one which were both masters of the materium and the inmaterium. Beings who could draw upon the power of the Warp and give the thoughts power and form. Beings which could move between worlds and across time effortlessly. Beings which would never need to fear the Xeno or even death itself, for the Warp transcends death and the greatest masters of the Warp - those who are called daemon princes - have become truly immortal. Imagine all of humanity ascending to this level.
Humanity already worships Chaos in its heart of hearts. An admiral gives an offering to Khorne with each battle he wins. A lover's every tender act of passion expression of affection is a prayer to Slaanesh. When every a scholar uncovers a new mystery, he is as a priest of Tzeentch. And all of gain the favor of Nurgle when we frantically struggle in the face of our mortality. No wonder the powers of the Warp are called "chaos" - you have chaos when people are at war with themselves and their own nature. You have destruction when people resist change and advancement. When we embrace Chaos, we embrace ourselves, we embrace a future where we can be better than what we are, we embrace hope.