Hannah single-handidly ruined the show
I'm not one of those people who think the show just flat out sucked after season 4.
It's been a while since I watched Dexter AT ALL, so I can't remember everything, but I liked seasons 5 and 7 quite a bit. I thought the stuff with Johnny Lee Miller and his psycho friends was awesome, as was Dexter finding a connection to Lumen; even if Stiles acting was weak.
And I really enjoyed the hitman stuff with Ray Stevenson.
From what I remember, season 6 was not good and 8 was flat for a final season.
But what really derailed the show as that God damned Hannah character. It made absolutely no sense whatsoever for Dexter to not only fall in love with her, but trust her with his child.
For starters, Hannah is a piece of garbage. Dexter has a code. It's well-established. He sometimes bends the rules, but at the end of the day Dexter does not intentionally kill innocent people. Hannah does. She did it without hesitation. First there was the girl she killed while on the killing spree with her old boyfriend. Then there was her ex-husband, who threatened to leave her over a disagreement on abortion. Then the author. Then she tries to kill Deb AND Dexter. She killed her mentor too, though I admit I forget if it was a mercy killing or she did it to get the woman's gardening business.
The show went completely off the rails once she was introduced. Pretty much no one liked her. She went against what we liked/defended about Dexter. Dexter killed bad people, but let this bad one live. We liked him, didn't like her, so what the hell are we supposed to feel when they end up together? We aren't rooting for her, but she's with Dex. It really added unnecessary conflict (and not good conflict) to the show. Good conflict was when Dexter was thinking of killing Doakes. Siding with Hannah? Inexplicable and bizarre.