I think Chaos can be done well or badly in the WHFB setting, there's some good fluff there IMO but it gets buried a bit too often.
To my mind, the point of Chaos is not so much that it represents "evil" or thinks of itself in those terms, but that it represents excess and the freedom to act on ones strongest impulses and desires - hence, chaos, the lack of restraint and order. The fact that complete and total lack of restraint tends to create bad guys is I think a reasonable trope, but well-written Chaos is a seductive rather than broad-brush evil force. It manifests best in the soldier whose love of war leads him to turn to a god who thinks of nothing but killing, the desperate wizard who looks at the corruption of the world and turns to Tzenteech in desperation for change, the hedonist who becomes a cultist of Slaanesh, the survivor of a plague-ridden village who turns in despair to worship the force of disease. The demons are sort of vectors for this, they are personifications of this lack of mental restraint that encourage it in others.
Chaos therefore wants two things; firstly, to fulfil the desires of the various cults as personified by the various demons, and secondly to destroy what they see as the shackles on the rest of the sentient races, namely their religions/social order/etc. I don't think a well-written Chaos is really out to annhilate the world in the way some of the WHFB undead are, but they'd like to see a total mess and breakdown of order and dominance of their various cults, demons everywhere etc. The sort of ultras who end up in the actual Chaos armies probably tend to be the most deranged, or perhaps a lot of the marauders have just been recruited en masse with their co-religionists and don't have any way of deserting!
So why does all this not come through in writing for WH? I think it used to more than it ended up doing so; after the Nemesis Crown campaign the writers seemed to be turning more and more to upping the stakes in everything, and ended up completing the journey from the old (say 3rd ed) world which was quite gritty and about continued struggle (including against our own and more literal demons) to a very millenarian end of the world scenario. Like, Storm of Chaos for example was big and rather doomsdayish but it wasn't actually the literal end of the world, just a major catastrophic invasion. The more things were ramped up into "are you on THIS SIDE or THIS SIDE" the more black and white they had to make it and the more you got Chaos just being written as world destruction in army book format.