Dexter Morgan is a Forensics Expert, a loyal brother, boyfriend, and friend. That's what he seems to be, but that's not what he really is. Dexter Morgan is a Serial Killer that hunts the bad.
After a season-plus stretch of Dex-Deb drama in which Deb discovered her brother’s secret, became his accomplice and fell in and out of love with him, she finally sank to his level in the Season 7 finale. In a harrowing and intense and climactic scene, Deb shot LaGuerta to save Dexter from being discovered as the Bay Harbor Butcher and herself from the infamy of being the homicide lieutenant who aided and abetted her serial killer brother.
Just as Dexter was faced with a choice last episode between Deb and Hannah, Deb had to make a choice of her own in the finale. With LaGuerta yelling “Put him down!” and “This is not who you are. You’re a cop. You’re a good person. You’re not like him,” Dexter put his arms up and invited Deb to shoot him to save her last shred of innocence. “Do what you gotta do,” he said calmly. But she was in too deep, and chose her brother — and career — over LaGuerta’s life. After she pulled the trigger, Deb rushed over to her former rival in a scene that Jennifer Carpenter apparently improvised, sobbing and screaming out “Oh my god” over and over again, in disbelief of what she had just done. At one point she looked at Dexter and mouthed, “I hate you.”
In retrospect, Deb’s character has been devolving to this point all season, and her co-dependent relationship with Dexter finally got the best of her. She has been steadily sliding towards her brother’s twisted code since the Season 7 premiere, when she helped him burn down the church where he killed Travis Marshall. She’s planted evidence, lied and covered his tracks. Shooting LaGuerta capped Deb’s tragic descent into Dexter’s bizarre and murderous world.
