So throughout the whole series they humanize dexter, and then with the final episode they just decide that dexter does not deserve happiness... I understand that he is a serial killer, but you have to know the message that you are sending throughout the whole show... I would have rather Dexter died from the storm than him faking his death to live alone...
They did humanize him, We have all felt at some point that our actions have hurt people in Dexters case he feels like the few people he loves have all died because of him. That is why he seperated himself from them. They just took those human characteristics to the extreme.