A lot of people have been saying this series is seriously treading water, that seeing a Serial Killer do what he does over and over is stale, well it's episodes like this that get me to keep coming back to the show!
Since LaGuerta figured out Dex was the BHB, we've all been expecting either she'd unravel completely and be drummed off the force, or end up on Dex's table. As I watched this episode I kept thinking 'he'll pin Estrada's death on LaGuerta' as a kind of botched frame-job on Dex, it would work with the planted evidence earlier in the show, but what they came up with excellent. Deb kills her to save Dex and after, it looks like the both of them are about ready to break apart from what the two have done and done to each other.
If the producers choose to end the series next season, it should be the best series ender in a long while.