I think it’s at least possible, though, for Karrin to be something of a sleeper agent. Remember: Dresden was, technically, a Denarian for four novels and never once drastically screwed things up for the Home Team. Not everyone is as stubborn and immovable as Dresden is, but it’s worth noting that having a supernatural brain-dwelling parasite doesn’t necessarily mean that you’re going to do what it wants. The fact that Karrin did not act in Nemesis’ best interest on the island is not unflinching proof that she’s not influenced by something.
Look at it this way: IF Karrin is Nemfected (which is a big IF – there’s a bunch of things out there that will give Power for influence) then it’s important to note that we have no evidence at all that Nemesis works in such a way that it has complete control over a person against their will. Lea wavered back and forth between personae. Cat Sith struggled and fought against the intruder. Maeve could be reasoned with, and treated her infection as a gift that gave power. The infected seem to have the ability to give up their will. Reclaiming will which has once been given seems to be a herculean task, but not impossible.
In her desperate bid to protect Chicago when Dresden was seemingly dead, she became an ally and employee of John Marcone in his Brighter Future Society organization, and commonly allies with creatures who think that humans are just delicious. Her old allies are getting injured and seriously hurt left and right. Is it so crazy to think that, if offered, she wouldn’t have picked up another weapon in order to do what needed to be done to save others? Especially if the one offering made it plain that whatever dark power being taken up would have no more influence over her than she allowed?
I think it’s at least possible. And if true, than it makes her reunion with Dresden in Cold Days even more compelling: because rather than speaking to him as an ex-cop who’s seen a lot of people go down that left-hand path, she’s speaking to someone who’s in the same exact precarious position that she’s in, and giving him the same advice that she’s trying to live by. Remember, Karrin told Dresden at the end of Cold Days that she works with a lot of monsters these days. What if one of the monsters was internal?
Yeah, it’s a WAG. But someone’s bound to be a traitor, and Karrin would be one of the biggest and most painful blows to Dresden, other than maybe Eb or Thomas. And while Ebenezer is still definitely the most suspicious and most likely to be a secret traitor, Karrin definitely has the potential.