(S04E10) "No one's ever going to hurt you again ... especially me." - Dexter to his infant son, Harrison

Every season, Dexter manages to learn something from every one of his various victims and kills: the origins of his "Dark Passenger," the need for humanity to have a silent hero, why you shouldn't share your serial killing secrets with your best friend.

This season's big lesson seems to be the deepest and most important of all: his family. Dexter kept Arthur Mitchell, the Trinity Killer (although after tonight, he should be redubbed the Trinity Plus One Killer since we learned that Lundy missed a crucial piece of his serial killings), on a long leash hoping to learn how to juggle his family mask and his private hobby, something Arthur has managed to do for more than 30 years. Unfortunately, the lesson he learned was a harsh one: your family is either a family or a mask and can be both. That and don't follow Arthur Mitchell's advice, ever.