I think that was exactly the point - after seasons of building up the idea that he could change and had changed, he gave up/realized that Vogel and the ghost of Harry were right all along when they told him he couldn't ever be anything other than a monster/killer. That dead look in the last scene was an echo of who Dexter was in episode 1, as though none of the events in between touched him at all.
As far as Deb goes, how he dumped her was a big part of my problem with it too - that he would put her, the person who arguably meant the most to him in the whole world, in the same dumping ground as the scum he killed, that he would deny her the honor of being laid to rest as an officer slain in the line of duty... That was out there even if you think back to Dexter in the first season. He didn't feel what others did, but he had a respect for what others felt and for the traditions and rituals of normal people/normal life. I kind of get what they were going for - he felt like he'd killed her, like she was another of his victims - but it still didn't ring true.