I was talking about Breaking Bad the whole time. If you didn't question this until that very last sentence, start watching now. It's Dexter with a fitting ending, fewer preposterous instances, and with tour de force acting. The only weakness is the first season drags on while it sets up all its plots and develops all the characters.
Excellent library Movies Online

White Collar, Hannibal, Bones, The Walking Dead, Breaking Bad
Heroes is another show with a serial killer as a main character, as is Hannibal. And so is Bates Motel, in a way. You might like any of them. Also, try Prison Break, Fringe or Alias.
Six Feet Under (HBO). The guy who plays Dexter is also in there
It's pretty hardcore, but I liked Oz. And I also thought the last episode of Dexter sucked.
Supernatural is a very good series to watch
Breaking bad!!! Or house of cards. Or watch the movie Donnie darko. I had the same problem. Those will blow you away
Breaking bad! Of corce
The Walking dead! It's on Netflix
Yeap so I just finished with Dexter (awesome series if I may add although I personally didn't find the ending to be as awesome) and I can't decide what to watch next, any ideas?
Preferably something on Netflix and/or HBO GO