I'm not saying you can just have him flip right there after getting to the cabin, story as presented. You have to introduce elements to the story that aren't there before to massage it along but the fundamental characters personalities to me it isn't a stretch at all.
Basically Dexter needs to "redeem" himself in some way before Doakes collects all the pieces. Save his life, save the life of someone close to him, murder someone Doakes really REALLY hates but can't catch, give Dexter some form of perceived leverage over Doakes where he doesn't think it's a good idea to go for it right away. I mean those are just really basic ideas off the top of my head, but the principle idea is that I don't think that Doakes conceptually would necessarily be against the idea of a vigilante if it wasn't him, so not immediately crushing him isn't out of the realm of possibility to me if the story was laid out a bit differently.
And because he appears to hate Dexter so much no matter what the option is you don't ever take it as a given he's letting Dexter get away with it, that's why it'd still be intriguing.
No problem!
This is a cute and pop genocide of love!