Over the years, "Dexter" has had a wide variety of characters and plot points that have polarized the fan base. You either loved or hated Lumen's arc, and the decision for Travis Marshall to be hallucinating the existence of an entire character was likewise divisive. But perhaps no development in the show's entire history has been as contested and detested as the decision for Deb to develop romantic feelings toward her adopted brother, Dexter.
The Deb-Dex relationship plot developed late in Season 6. Deb was seeing a therapist and discussing how all of her past relationships had failed because she was attracted to unsuitable men. Her therapist suggested that Deb might be attracted to unavailable or unacceptable men because the man she really wanted to be with was also a socially unacceptable choice.
Deb seemed mortified at the idea of being in love with Dexter when it was brought up, but by the season finale, she was driving out to see Dexter and tell him about her confused feelings for him.
In Season 7, Deb now knows the extent of her adopted brother's murderous tendencies. And while they affected her feelings at first, Deb has now confessed her feelings to Dexter, who is puzzled by them and seems unlikely to reciprocate them.
Dex is with Hannah McKay this season, a woman who seems to truly appreciate all of Dexter's qualities. Even if Hannah is taken out of the picture for some reason, the Dex and Deb relationship that's been hinted at seems unlikely to happen in a way that would please fans.
The simple fact of the matter is this: Very few "Dexter" fans liked the idea of Deb developing feelings for Dexter when it was first introduced. While the scenes where Deb has wrestled with her feelings for Dex have been well-acted in Season 7, it is still puzzling that the producers and writing staff kept going with this storyline, despite widespread hatred of the idea from the fanbase when the idea was introduced in the previous season.
But the core reason for fans hating Deb's emotional "revelation" has little to do with the creepy incest undertones, and far more to do with a fundamental change in her character. Deb, though unlucky in love, has always been a strong character with a strong familial bond to her only remaining family.
It's almost insulting to the character of Deb Morgan to suggest that she needs a romantic reason to not turn her brother in. Manufacturing Deb's romantic love for Dexter out of nowhere seems sloppy, like the producers and writers wanted an easy way to explain why Deb would be loyal to a brother who is a killer. It was an unnecessary step: Deb has already been shown to be loyal to Dex.
The cardinal rule of making good TV is simple: Make things as difficult as possible for your characters. By making Deb have romantic feelings toward Dexter, the show tried to add dramatic tension but ended up doing the opposite.
Deb's feelings for Dex actually ensure that she won't reveal his secrets (at least, not right away). But if she had never developed those feelings, fans would have been on the edge of their seats since the season's first episode, just waiting for her to snap and tell Miami Metro about her brother's nasty little habit. A show where Deb hunts Dexter will always be more interesting than Deb simply being her brother's accomplice.