I have seen two thirds of the first series of Dexter so far. It's dark and violent but it's also dark and funny. Screenwriter/producer Drew Z Greenberg used to work on Buffy the Vampire Slayer and I see a lot of similarities between the two shows.
Dexter is a serial killer, but at the same time he is a nice guy with a moral compass. Not unlike Angel and Spike who both were created as killers but were forced (Angel) and chose (Spike) to change. Like the two vampires, Dexter struggles with right and wrong and seems to have equal trouble resiting the urge to kill and maintaining relationships.
Both shows also have plenty of dark humor. In one episode the Miami police discover a pile of body parts wrapped in brown paper arranged in the goal of the Miami Blades ice hockey stadium.
Detective: – What do you think he's trying to tell us?
Dexter: – That hockey's a violent sport?
Also, actress Julie Benz who portrays Dexter's girlfriend Rita played Darla on BtVS and Angel.