1)
Thor's Plan doesn't actually need the Dark One; just his power, which Redcloak can channel. Provided he has a 9th-level slot available near the climax, Redcloak can actually turn against his deity to help save the planet. He probably won't be granted any
new spells after that, if not lose his powers and status entirely - but he'd still be a hero.
2) Narratively, Redcloak's entire character is built around Sunk Cost Fallacy - feeling he has to stay the course because all the bad things he has caused for the goblin people (including his family) along the way would "be for nothing" otherwise. So for him to finally realize that, no actually, anyone can choose to change who they are and start being a better person at any time no matter what they did before, would be a satisfying direction to take his narrative arc and a good lesson for the fiction to impart.
3) The Dark One himself seems a bit more sinister than the comic is letting on; sure, his own dogma paints him as a champion of the oppressed goblinoids who ascended due to the world's sheer injustice, but we only have his word for it that that's how things actually went down. Moreover, his actions since then (including but not limited to his rather cold treatment of his own high priest, and not seeming to care very much about goblins sacrificed to the plan) have not exactly lined up with the supposedly benevolent image the goblins have of him. In a sense he reminds me of Shar, duping his worshipers into believing she has their best interests at heart more than every other deity while in reality, discarding them the moment they're no longer useful.