I am not sure why there is such a ground swell of negative sentiment to both this finale and the series as a whole. In my humble opinion, this has been the best series of Dexter to date and epitomises the benefits of character development.
Dexter's progression from cold-hearted serial killer to angst driven vigilante has mirrored Julia Stiles character's descent into darkness. Their transformations crossing paths over the last two episodes sated that quintessential human requirement of acceptance that Dexter so craves, acceptance that someone knows what he is but they have not turned away from him. The fact that the excellent Julia Stiles lost her "dark passenger" once revenge had been accomplished and in fact started her ascent back to light has once again ripped this possibility of a normal existence away from Dexter. His feeling of sadness was palpable in the final kitchen scene with Stiles and this magnified the power of his final monologue and ending scene. Has Dexter finally accepted that he will always be alone.
I think some viewers missed the point of Lumen indicating her wish to return to normality and to abandon Dexter. Some suggested she even used Dexter. While this is true in a basic sense, she ultimately provided a catalyst for Dexter to reconnect with humanity. Her circumstances were not normal and it made sense she formed an allegiance with a similarly broken person. The natural response after fulfilling her vengeance was to reject the darkness or indeed look away from the abyss (to paraphrase Nietsche), to Dexter's detriment. What is painfully clear is that with Lumen no longer there, what/who will provide Dexter with the necessary mechanism/plot device to accept his humanity (which he clearly does not believe is there), if that of course is the ultimate aim of the show. If that is not the aim, I fully expect a dark and brooding Dexter to reappear in the coming seasons which would ultimately define this as a tragedy as he would have come full circle.
In the end, this was an incredibly powerful season and comes straight after the brilliant season 4. Where to from here is anyone's guess but I will be waiting with bated breath.