Some stuff I was discussing wiht a friend:
Iit was about character development, about reality, socialization, outcasts. It was about struggling with two worlds and whether it's possible to appear normal when you're not. The ending shows us that the way Dexter truly lived his life, destroyed everyone around him.
The code was his grounding and that's gone. Besides that all he really had was Deb to ground him to that idea of Harry that forced him to try and be humane. After losing that his world did collapse. And although he loved Harrison in his own way, Harrison was a result of another attempt at a cover life an human connection failing and I don't think he wanted to face that anymore. He needed to be alone and go against everything Harry and voegul wants because they were humane,not him. He questioned constantly whether being alone was the only option because he either never fully could connect or destroyed people when he did.
People seem to think Dexter transformed into some amazingly emotionally deep and empathetic person as the show progressed, but he really didn't. He had some minor emotions now compared to absolutely nothing, but he didn't understand them, couldn't really identify them, didn't know how to express them, and really couldn't really others emotional depth and respond to it appropriately either.
The deep trauma he suffered as a child completely altered his brain function and he will never ever be able to connect or interact, understand or have the same emotional capacity as regular people because of that..
People forget how much he fakes. It's easy to trick yourself into thinking something is real when you've faked it for so long and you really want it to be real.
and i think this ending was the dark passenger fully returning, and he is now just a vehicle for that. with no code, and no connections; however false, to tie him to any shred of humanity.