I haven't seen season 5. But seasons one through four were about Dexter choosing between someone (brother; lover; friend; hoped for mentor; seasons 1-4 respectively) for his killing self and the people in his regular life, (Deb, Rita/Astor/Cody/Doakes; Harry, in the form of his Code; Rita/Cody/Astor/Jonah, seasons 1-4, respectively.) He always chose the people he loved. The story was getting a little old, because he didn't move on. The series has been repeating the same story, very satisfying first time, but progressively less interesting. Except that his unfailing success in murder is getting more and more far fetched.
The problem with killing Rita is, in season five, when faced with going back to his killing ways or moving on, Dexter now has less to choose between. The sudden appearance of fabulous grandparents on Paul's side means Astor and Cody are leaving, and Harrison, as a baby, is a huge sacrifice. Being a single father is rough.
So I would have hoped, Deb finds out, and Dexter is arrested, killed or suicided. I haven't religiously avoided spoilers but although I don't know what season 5 really has, other than a cast list, I'm pretty sure this wasn't addressed. So, I hope they retcon Paul's parents into a grave or the slammer or California. And Dexter starts trying to teach Astor and Cody the Code (albeit modified.) Then he starts coming to terms with the fact that Harry was crazy and emotionally abusive. And Deb, trying to help with the kids, finds out about Harry's abuse. It is past time for Deb to be more than a 4F character. (Foul-mouthed Fool but Free with her Favors.)