I don't know where this comes from, but a much older Warhammer fan than I once told me that the oldest fluff regarding the Ruinous Powers is that they were responses to the horrors of life. Essentially, life sucks, and ...
- Tzeentch says, "That's true, but things can change!"
- Nurgle says, "That's true, but it only hurts you if you let it."
- Slaanesh says, "That's true, but you can still take care of yourself."
- Khorne says, "That's true, and it's okay to get angry about it!"
I don't know if this is super old fluff or something my grognard friend made up, but I've always liked this view of Chaos because all of these are intuitively acceptable responses to the horrors of life. That's what makes Chaos so attractive, particularly when juxtaposed against an Imperial Creed that tells you that this is the best of all possible worlds, and if you think things are bad it's essentially because you lack faith. Sometimes things are broken, and it is okay to get mad about it. The tragedy of the four Ruinous Powers, in this view - the reason their most advanced disciples end up gibbering impotent idiots - is that none of them can accept any response other than the one that birthed them. It doesn't matter what the evil is; if the Chaos gods' response is the only one you have to evil, you end up a monster.
Suppose the evil you're reacting to is something as clearly evil as institutionalized racism. The Imperium tells you that there is no racism here. Khorne tells you that yes there is, and furthermore, it is right and good for you to be angry about it. But if you are only driven by your anger, you end up knee-deep in the corpses of white people, frothing at the mouth and screaming, "I AM CORRECTING INSTITUTIONALIZED RACISM!!!!!!!" That's true no matter what you are responding to, be it Imperial bureaucracy, the fact that your wife left you and your dog died, or the fact that the dark eldar are raiding your settlement and there's nobody but you standing between them and your wife and kids.
The other thing I like about this "mono-response to life's horrors/evils" view is that it makes Abaddon's project seem more sensible. None of the Chaos Gods present a viable alternative philosophy by themselves, but taken together, and balanced in moderation, they pretty much are the intuitively correct response to life's horrors and evils. A sufficiently vain man could think (or be tempted into thinking) that if only a strong-willed individual could hold the philosophies of the Chaos gods in balance, he would have a humane, humanist alternative to the morally bankrupt dreck that the Ecclesiarchy and the space marine chapters peddle.