Because Dexter has the psycho Daniel Vogel on his table, in a perfect last kill, all set for Dex and Hannah and Harrison to make way to Argentina, and Dex instead calls in Deb to arrest Daniel all legal-like. That's not a bad plan, since Daniel can also be credited with all Dex's killings, but it's not as good as the original plan.
Because Marshall Cooper - having been tailing Dexter - walks in on the bound Daniel, as Dex and Debbie are saying goodbye outside, and Cooper cuts him the loose. The immediate result is Cooper looks like he was stabbed to death by Daniel, a very good thing. But the next result is Deb is shot by Daniel - using Cooper's gun - and she looks to be very seriously wounded.
Once again, unneeded coming attractions tell us Deb will at least make it to the finale, and I sure hope she survives, but that looks far from definite. She has totally come to terms with and accepted her relationship with Dexter, she's getting back with Quinn, so her not surviving would be especially terrible.
Other than Dexter not being his old, savoring-of-righteous-murder self putting Deb in this position - and everyone else that Dex cares about, with Daniel on the loose - this episode has a brilliant collection of scenes with the new Dexter saying goodbye to his old friends.
This includes Quinn, Batista, and, in some ways most touchingly and significantly, Harry, who realizes Dexter doesn't need him any more. And since Harry is a part of Dexter, that was certainly the most eloquent way of Dexter leaving his old self, saying goodbye to the inner Harry that was essential to the survival of the old Dexter.
Will this new Dexter - lovable and loving - survive the finale? I'll be on tenterhooks until next Sunday. It's asking a lot of I don't who for both Dex and Deb to survive - and Dex to live dangerously ever after with Hannah - but I'm hoping.
And see also Dexter Season 6 Sneak Preview Review ... Dexter 6.4: Two Numbers and Two Killers Equals? ... Dexter 6.5 and 6.6: Decisive Sam ... Dexter 6.7: The State of Nebraska ... Dexter 6.8: Is Gellar Really Real? .... Dexter 6.9: And Gellar Is ... ... Dexter's Take on Videogames in 6.10 ...Dexter and Debra: Dexter 6.11 ... Dexter Season 6 Finale: Through the Eyes of a Different Love
And see also Dexter Season 4: Sneak Preview Review ... The Family Man on Dexter 4.5 ...Dexter on the Couch in 4.6 ... Dexter 4.7: 'He Can't Kill Bambi' ... Dexter 4.8: Great Mistakes ...4.9: Trinity's Surprising Daughter ... 4.10: More than Trinity ... 4.11: The "Soulless, Anti-Family Schmuck" ... 4.12: Revenges and Recapitulations
And see also reviews of Season 3: Season's Happy Endings? ... Double Surprise ... Psychotic Law vs. Sociopath Science ... The Bright, Elusive Butterfly of Dexter ... The True Nature of Miguel ...Si Se Puede on Dexter ... and Dexter 3: Sneak Preview Review
See also about Season 1: First Place to Dexter