Enjoy the plunge, but be comforted that the Harry you fell for will rise again.

I think the end game is for Harry to get back to his values. The core of Harry is still going to be there, but temptation and gradual corruption are themes too strong to be written off when looking ahead. Butcher often says in interviews & Q&A sessions that the plan was for Harry to take up one of the evil powers presented in order to rescue Maggie, but until he set out to write "Changes", he did not know whether he was going to have Harry summon Lasciel's coin, conjure up the Dark Hallow or accept Mab's offer. And from everything I have seen of Butcher, it seems very unlikely he would develop three elements like that, only to toss two of them out forever. I think he only was ambiguous as to which one Harry would take up FIRST, that all three were coming up again as choices, so the question was, which one had more storytelling opportunities in the interim. While he could have used the Denarian Order to make Harry do the same stuff Mab does in books 14-17, they're so overtly evil that it would be hard to justify Harry's cooperation. The Darkhallow lacks an institution, unless maybe it somehow puts him under the authority of Cowl & Kumori or Drakul & Mavra. But again, the problem is cooperating with obvious supervillains. So it makes sense for Mab & the Winter Court to be the vehicle for frog-boiling Harry into villainy. Working with Winter (and now Lara & the White Court) is going to make it easier to rationalize working with increasingly worse allies, especially now that the White (and Gray) Council won't be taking his calls anymore. And Nicodemus has dropped too many hints about working for a greater good and believing he can talk Harry around to his perspective, not to mention similar things from Cowl, and Drakul apparently also being a starborn... I think we're going to get stuff in their background like Mab's secret purpose as the defender of reality against the Outsiders that makes working with them tenable, just like working for Mab is a more important job than we thought when she first offered him the Knighthood back in book 4.

Evil Harry is not the end game, it's the penultimate boss fight, building to his equivalent of a Dragonmount moment. I think ultimately, that Amoracchius, which he has not issued except in "Changes" for what he would have seen as the most extreme emergency of his life, withholding even for the Battle of Chicago, is going to be Harry's to wield, after he rejects or foreswears all the evil or corrupting powers and resources he has accumulated by the end, just like Rand got rid of the Choedan Kal and took up the Sword from the Stone for the final conflict. Like WoT, I think ultimately, Harry's destiny is for a moral conflict rather than a physical one. It's why he has some of the same traits as Rand, like stubbornness and a refusal to go along with the establishment authorities' version of right and wrong, or chivalry toward women, and an obsession with protecting friends and loved ones, often to the detriment of that actual goal. These traits establish someone as possessing a moral center, as opposed to being followers, that they have the will to hold onto or return to their core vales in spite of blandishments or aberrant acts, and selfless motivations, to lessen the impact of their corruption and make their redemptions plausible.

I do think that Harry's still going to be his fun self for the most part, because that's the series style, while WoT had other characters to lighten the mood as Rand went darker. Maybe he'll be Bryonic and miserable throughout the book where he eventually has his 'road to Damascus' moment, but not until then, and I think there's going to be a lot of fun or funny stuff in the books to come as well. It's just that the serious, character-exposing moments are going to be more darkly tinged.