Hello friends, I've spent the past few days casually going through a few dozen pages of discussion. I knew this was a contentious topic, but the exact nature of it is still fascinating to read through.
Personally, and this is just a relatively new fan's impression, I agree with the idea that Chaos being Ultimate Evil is just a really weird position and not really conducive to the long-term health of the lore. There should be
some in-universe reason for people to turn to Chaos that's not just 'ohhhh power ohh mind influence'.
On the whole, they should be somewhat more nuanced than 'look theyre so bad that every evil the imperium does is justified, kinda'. It could be, as Night says, that they have elements which are essentially just a mirror held up to the Imperium but reveling in their terrible nature rather than denying it. But it should be more than that, there should be enough nuance within what we call "Chaos" that it has heroic or at least sympathetic elements, or people who are there because there's nowhere else that'll take them.
For instance, in the abstract, I can see ways for people to turn or be corrupted towards the horrible aspects of Khorne or Slaanesh or Tzeench. You start off by talking about the more positive aspects like athleticism and honor, self-expression and love, hope and ambition, etc. I can think of scenarios in which an ordinary person starts worshiping them and doesn't immediately turn into a horrible person.
Part of the difficulty here is that, as has been noted before, it's very hard to see any positive aspects to Nurgle, any real reason to worship him if you're not already within the cult. He'd be sort of ok if the Chaos Gods had a Four Horsemen of the Apocalypse deal going on (Khorne would be a good shoe-in for War), but they really don't and it's just weird.
... I think one reason the thread's been spinning its tires for years is that you need to change so much that you're not talking about 40k anymore. You're actually writing your own custom setting about extremely weird demon mutant rebels and passing it off as an existing setting, which feels very... improper.
I see this a lot in other big fandoms, honestly. Arguments about 'hey how do we make things... different' tend to be answered with 'you cannot, thats not how it is canonically because of X Y and Z in-universe things'. Thermian arguments all the way down.
For instance, Alignment in D&D. In its universes, morality is objective and there's mechanics tied in to it; there's evil planes of existence, evil races dominated by evil gods, and a lot of the rhetoric surrounding it, the conceits involved, really are just thinly veiled white supremacist talking points. You have eeeeeevil and literally black elves who have a 'matriarchal' society even though it holds to patriarchal ideas, you have dragons divided on whether they're good or evil based on whether their scales are shiny or not, and really a lot of other stuff that's not this thread's point.
A lot of resistance to changing settings really is just a knee-jerk 'well this is how things have always been, why would you change them?' reaction.
