Season 7 features Call Backs and Continuity Nods to every previous season...except for Season 3. Or maybe it did include that season after all, in a more implicit way. Consider how Season 3's villain, Miguel Prado, deliberately turned away from the Code and plotted to murder LaGuerta in order to protect himself... Sound familiar?
In season 3, Camilla has terminal lung cancer and asks Dexter to kill her. When speaking to Deb about it, Deb mentions that if that ever happened to her, she'd want someone to pull the plug. In the series finale, Dexter does just that when Deb is a vegetable. Dexter isn't really killing Deb in his typical way, he's just carrying out her wishes from 5 years prior.
The Koshka Brotherhood’s name. Koshka literally means ‘she-cat’ in Russian, which comes across as somewhat Narmy, but remember that they run strip clubs and work in human trafficking—it’s not ‘Kitty Brotherhood’, it’s ‘Pussy Brotherhood’.
As if Rita's death wasn't heartbreaking and horrific enough. When you start to think of what a sadistic freak Arthur Mitchell aka Trinity was, just imagine what he said to her. Poor girl had no clue why she was dying... and her baby had to watch.
What's worse is that her death didn't fit the pattern: she was a mother of three, not two, and mothers were pushed off buildings. The daughters were bled out in bathtubs. So while it could have been him, it could just as easily be an unknown copycat killer.
While we know that Dexter's code keeps him from killing innocents, and he is able to control his Dark Passenger enough that he is not a danger to those around him, in season 2, he tells Lila that he keeps himself at a distance from the people he cares about for fear he might "hurt them... like I've hurt so many others." Dexter has actually considered the possibility that he could unleash his dark passenger on his family or co-workers. This comes to a head in the first season finale.
At the end of series 2, Dexter holds Doakes captive and has long conversations trying to convince him they're Not So Different, it sometimes comes across less as justifing himself for killing people and more like trying to justify killing Doakes and cutting him up. And as Dexter kills a drug dealer after very little research in his preferred manner right in front of Doakes, you can assume Dexter was feeling very stabby at the time.
In Series 5, One of the serial rapists has a tendency to stick three fingers in the mouths of his victims. Creepy enough even before you realise that this guy's other distinguishing feature is that he is a dentist. A children's dentist. He puts those fingers of the mouths of children on a regular basis.
Dexter's kills are not at all like the quickie knife-in-the-chest kills of his TV counterpart. They're slow, torturous vivisections that cause the victim to die in the most agonizing way possible. But hey, the victims are murderers so its just karma right? Until you remember that Brian kills in exactly the same way, and his victims are prostitutes who's only crimes was being reduced to selling themselves to survive.
Not to mention there's no mention of Brian being asexual like Dexter, so what're the chances he rapes them first?