The lore is extremely strict on Chaos. The reasons people follow or join a faction are colorful and interesting, but nothing that can translate into any of the games.
Hence why we can’t have Chaos heroes in a Vermintide game without it being immersion breaking.
Like there’s no such thing, apparently, as a human mage that can dabble in different schools of magic without exploding, there’s no such thing as anyone ever sprouting a tentacle and going “You know… this is a bad idea. I would like to quit Chaos.”
Heck, Unchained is actually pure fanon, according to FS, themselves, they had to work hard to get that class approved since anyone brimming with that much fire magic would have already exploded.
I mean, technically… she did summon Slaanesh into the world during the End Times to consume all the elven souls, which is why they were all brought to the Age of Sigmar-verse to be belched out and reforged by the new elven gods.
You can’t really say that was willingly, though. She did it out of pure rage, despair and madness and didn’t really know that summoning Slaneesh was the outcome (allthough, what else?). She just unleashed her magic upon seeing the death of Tyrion and (supposedly) Malekith right after.
In the End Times all the elves banded together, so Dark Elves, High Elves and Wood Elves were counted as one faction (with the exception of the Aestyrion), and that’s what the three careers of Kerillian represent, and why a Druchii would be counted to the ‘good’ forces of the World.
All the ‘bad’ forces were basically Chaos and the armies of Khaine, with everything else, even Nagash, trying to halt their advance.