Dexter is a work in progress, like all serialized dramas. The track record for them is extraordinarily poor. It is necessary for the dramatic resolution of the story to be artificially delayed. Thus, Dexter cannot go finally, completely sane til the end of the show's run, even if it means
that the ultimately logical consequence of rejecting Rudy's acceptance of him as he is must be delayed by creating a lover who, remarkably, will accept him as he is but who---just like his brother require that he reject his decent life, even to the conveniently melodramatic extreme of making it a choice between someone's life or death. And yet again, the choice of his sane life means doing his same old work! What an amazing coincidence!
As can be seen, Dexter, like so many character driven shows (aka soaps,) fallen to telling the same story again. But not all hope is lost---Dexter has found out not just that Harry lied to him but regretted what he did to Dexter. He hasn't had time for that and what it means to really sink in yet. Understanding that Harry was nuts is an inescapable part of his recovery. The thing is, if he doesn't realize it sooner or later, the show will just be whether a charming serial killer can have it all---sister, girl friend, meaningful work and a fun hobby.
Having the show turn out to be the escapades of Dexter the same at the end as at the beginning would be a complete failure artistically. I suppose part of their problem is that a man going sane seems like a happy ending, a comedy. (Dexter has lots of humor.) But both the plausible and morally conformist endings would be pathetic or tragic.