10 Small Details You Only Notice Rewatching Dexter
3. Sin Of Omission
We have already established a number of times how the series ends and that is with Dexter performing a mercy kill on Debra while she is in a coma. She unwillingly described the exact scenario two seasons earlier while talking to her therapist, Michelle Ross.
While in the process of their discussions, Michelle is instrumental in making Debra realise her love for Dexter - which is at least a little bit uncomfortable, as they are foster siblings. There is this exchange in "Sin Of Omission", season six, episode eight while they are discussing the latest victim of "The Doomsday Killer". Travis is revealed to be the killer in the end and the victim in question is his sister. Debra says words to this effect about the situation:
"And I think she was trying to protect him, which is what a good sister does. And then, he ends up killing her."
While the way that Travis and Dexter are responsible for taking the life of their respective sisters is vastly different, there is no denying that Deb went out of her way and risked a lot to protect her brother and he did indeed end up killing her.