Was Redcloak evil since the beginning?

[Peelee]

If you are correct, it raises the question of why does Stickworld differs from D&D in this regard? Why is augury, a 2nd level cleric spell that all clerics get, not commonly used in Stickworld?

It doesn't because that answer is obvious. It makes for a better story. As the author has said about other spells, such as True Resurrection. Spells that may make for fun game play may not make for good narratives, and this is a pure narrative and not a game.

ETA:

Is there any particular part of the D&D 3e rules that "gets in the way" of your story more consistently than others? I mean, it doesn't really get in the way because it's swept aside the moment it becomes inconvenient, but I hope the question makes sense.

True Resurrection, without a doubt. It's literally impossible for a mortal character to ever be completely out of the story because of its existence. Actually, all forms of resurrection are kind of a pain in the ass, though the other versions have roadblocks you can throw in the path. But because True Resurrection exists, every character death is met with, "Well, they could still come back!" forever.

Also, Teleport. Characters who blip right to their end destination do not for an engaging journey make.

In other words, any move away from D&D rules will be to make a better story