doctorbadwolf
Heretic of The Seventh Circle
So maybe evil-worshippers can't be PCs - but they definitely might be enemy NPCs. Because if evil is real, is present, and is powerful, somebody will worship it.
Similarly, the game might be not about evil organizations - but evil organizations are likely to exist, if nothing else as enemies.
I also now understand what you meant by "grimdark", but "not doing it" is not really an option, IMHO, if you have a setting like this.
If the PCs, however much they might be holy servants of good, have to eradicate a coven of worshippers of evil (or an infestation of evil possessions, or an avatar of evil that hides within an overpopulated urban neighborhood, etc.) that will be a grim and dark business, no matter how clean and bright the heroes are.
That is my opinion, of course, but that's what you asked.
A quick read on what grimdark means. https://en.m.wikipedia.org/wiki/Grimdark
TLDR, it’s a tone of story in which no one is “good”, or if they are, they’re either a dupe or a tragic fool destined to a bad end from which no good comes.
So, delving into the darkness to fight evil isn’t grimdark. A version of this sort of game where there are no true heroes, the Church is corrupt, the evil guys aren’t really that much worse than the good guys, and no one’s hands are clean, is grimdark.
None of the missions you describe require killing innocents, or anything like it. It might involve having to fight possessed innocents in self defense, but there’s no reason that a Romantic Heroic Fantasy game can’t include rules for subduing rather than killing. Also, even if the innocent dies in the process of being fought off by Paladins while the Druid of the Tuatha performs a ritual to bind the evil spirits, and the priest of the Light prepares to sanctify the area to drive the bound spirits back into the Void, that is something can can be dealt with without focusing the game on the gritty implication that the Paladins aren’t actually good or whatever.
The Romantic and/or Heroic Fantasy Hero can atone or otherwise deal with grief without making a grimdark game. There’s no need to focus the game on that.
@Bagpuss I don’t care. Find someone else to show off how edgy you are to.