This is the Way the World Ends
Dexter Season 6 Episode 12 (12/18/11)
Season six of Dexter ended with Travis on Dexter's table, right after Deb told her therapist that she was in love with her brother and just had to tell him.
So cut to the abandoned church, where Dexter is plunging the knife into Travis' chest, just as Deb arrives on the scene and witnesses this horrific, shocking scene. The final words of the season, courtesy of Dexter seeing Deb see him commit cold-blooded murder?