When trying to deal with the news that her brother is a serial killer in Dexter season 7, Deb has a nightmare while in the bathtub where she imagines Dexter as three notable serial killer villains from the series. Each season of Dexter introduces a new villain, which, aside from season 2, happens to be a serial killer that Dexter Morgan has to outmaneuver and put on his own table before the police department can apprehend them. Some of the killers take higher tolls on Dexter than others, such as The Trinity Killer and The Ice Truck Killer, but they also deeply affect others on the case, like his detective sister Debra Morgan.
Throughout the course of the TV show, the person Dexter struggles to hide his dark passenger from the most is Deb. After the death of their parents, both having issues with forming relationships, and working together at the police department, they became each other’s closest ally. With such a close relationship and Deb even momentarily being in love with Dexter, finding out that he was a serial killer and having to hide the information went against her morals and broke her. She had a breakdown in season 8 that led to her leaving the police department and Miami, but it all really started in Dexter season 7 when Dexter told her he was a serial killer and that their father, who is now Dexter’s “Dark Passenger,” helped guide him down that path.
In Dexter season 7 episode 4, “Run,” Deb is still extremely angry with Dexter for both being a serial killer and lying about it all their lives. Not only is she angry, but she’s also partially scared of what he is, knowing the terrifying nature of all the serial killers they’ve hunted over the years. The episode shows Deb taking a bath to unwind, but the relaxation turns into a gruesome nightmare where her fears of Dexter begin to take hold. She imagines herself as Rita, bleeding out in the bathtub after The Trinity Killer murdered her in season 4, then pictures Dexter in a suit as he asks her to marry him just as The Ice Truck Killer had, and finally sees her brother holding the machete that The Doomsday Killer used before she quickly wakes up.
Deb’s reference to The Trinity Killer is the most obvious with the bathtub setting. Trinity’s M.O. for his kill cycles always began with killing a woman and having her bleed out in the bathtub, mimicking a suicide. This specific kill seemed to be the most traumatizing for Deb as it’s how her sister-in-law was murdered, with Deb even blaming Dexter for her death after the nightmare. While not the most personal serial killer that Debra had hunted over the years, she still pictured Dexter as The Doomsday Killer after discovering his identity. The purpose of this reference was more for its recency, with DDK being the last killer caught in Dexter season 6 while, to augment the connection, she also first found out Dexter was a killer when she saw him stabbing DDK on the table with the machete.
Aside from the Dexter discovery, the serial killer that got to her the most was The Ice Truck Killer/Brian Moser, who used Deb in Dexter season 1 to get closer to Dexter. After meeting him in the hospital as her doctor, Deb and Brian (as Rudy Cooper) began seriously dating, with him even proposing to her before revealing his true self and trying to kill her. Although Dexter found out back in season 1 that Brian was his biological brother, Debra didn’t find out until Dexter season 4 when she began digging up information on Harry Morgan’s past confidential informants. Her traumatizing experience with Brian/Rudy/The Ice Truck Killer in Dexter was the most personal and was only made worse when she realized he was Dexter’s brother. With Deb’s fear of Dexter being augmented by his familial connection to The Ice Truck Killer, he may have benefitted by telling her that he killed Brian for her, which would humanize him more and show that he doesn’t always give in to his urges.