The Culture v's 40kverse

[The Glyphstone]

What the Culture would need to worry about is actually meeting a Chaos cultist. The Culture doesn't seem the type to kill on site and that can give the cultist the chance to communicate and slowly convince Culture members to 'just give it a shot'. Which can be devastating depending on the who it is.

Particularly if the Minds let it go on for a bit. They're obviously (and mostly deservedly) assured that nothing in this galaxy poses any military threat to them. If they're actually presented with a genuine Chaos cultist, a definitive 'point source' for the nebulous Outside Context Problem that Chaos has been up to this point, they might stay hands-off just to see what will happen, in a how-much-harm-could-it-cause sort of way.

Heck, the end result of all this might be a Chaos-Culture 'alliance' (more like benign parasitism), rather than an Eldar-Culture alliance. While Chaos is malevolent and self-serving, ultimately it still needs the existence of humanity/humanoid societies to perpetuate itself. If the Minds can render themselves Chaos-proof and able to block involuntary corruption, preserving the true essential element of Culture society, Chaos-worship might just end up yet another potential leisure prospect or recreational activity, if one that you can't really give up once you've started it.

I really like the idea of a legitimately rational sci-fi civilization trying to deal with the WH40K universe, but I feel like your Fic here isn't really "getting" the idea of GrimDark.

I think he knows that and is overriding it. It's like the 40K versus Gurren Lagann argument from before - the real fight isn't between the galaxy-scale robots and the gothic space cathedrals, but between the two presiding themes of grimdark futility against hot-blooded determination and willpower...they're mutually incompatible, so whichever universe's "rules" take precedence will ultimately reign supreme. Here, it's fairly evident that as the story is being written from the Culture's perspective, the "SCIENCE!" paradigm of the Cultureverse is being put forth as pre-eminent. They're the 'protagonists' of this piece, so to speak.