Warning: SPOILERS for Dexter: New Blood episode 7.
The Trinity Killer’s cameo in Dexter: New Blood rehashes Harrison’s most traumatizing moment, and is made worse by recalling that being naked was part of his M.O. Arthur Mitchell, aka the Trinity Killer, was the main villain in Dexter season 4, who Dexter got to know personally before he killed his wife Rita in the season finale. The Trinity Killer was known for killing in cycles of four, each of which had very specific rituals: First, he would kill a young woman in the bathtub by slicing her femoral artery, then he would force a mother of two to take her own life by jumping from a high building, then he would bludgeon a father of two, and finally kill and bury a 10-year-old boy. While Rita didn’t completely fit the M.O. for his first round of kills, she was murdered in the same ritualistic way in a bathtub.
Although John Lithgow's Dexter: New Blood return wasn’t a surprise, viewers were left in the dark about how his menacing Trinity Killer would appear. At the end of Dexter: New Blood episode 7, Harrison has flashbacks to the moment his mother, Rita, was murdered by Trinity in their bathtub and he was left in a pool of her blood. After killing Rita, a naked Trinity Killer walks over to the infant Harrison on the floor and comforts him by saying Dexter will be home soon. Confirming what Dexter feared since the season 4 finale, Harrison remembered every detail of Rita’s death, including the strange detail that Trinity was naked.
While Trinity wasn’t naked every time that he killed, he was always nude during the first kill of his cycle when murdering a young woman in the bathtub. This was significant in his M.O. for this part of his kill cycle, as he would enter the bathtub alongside the woman. As shown in Dexter's original series, Arthur would typically enter the woman’s home, fill her bathtub, and wait nude for her to find him, after which he would sit her in front of him in the tub and slice her femoral artery while holding her. Since Rita’s murder was never actually shown on-screen in Dexter's original series, it was unclear beforehand whether Trinity completely stuck to his ritual while killing her. Dexter: New Blood’s Harrison flashback finally confirms that this is exactly what happened to Rita in Dexter season 4, and answers why Trinity was completely nude while covered in her blood and speaking to Harrison.
Although Arthur Mitchell being naked while killing Rita seems to only be due to his also being in the bathtub, it actually has a much deeper, more disturbing meaning. The entire M.O. of Dexter's Trinity Killer is based on recreating the deaths and traumatic moments of his family, beginning with the accidental death of his older sister, Vera, when he was 10 years old. Arthur had spied on Vera while she was showering and when she spotted him, became so startled that she slipped and fell through the glass door, leading her to bleed out after a shattered piece sliced her femoral artery. Much more than his mother or father, Arthur was extremely fixated on Vera’s death for the rest of his life, especially since his own actions indirectly led to her death. Vera was found completely naked in the blood-filled bathtub when she died, and Arthur's M.O. attached himself more closely to this moment by placing himself naked in the bathtub as well.
There’s one aspect of Trinity’s ritual that is still unclear as to whether it was repeated in Rita's Dexter season 4 murder - using a mirror to watch her face as she dies. In Dexter: New Blood’s episode, Harrison recalls seeing Trinity exit the bathtub covered in blood while holding the straight razor, though it doesn’t appear he was holding a mirror. It’s possible that seeing Harrison scream and watch the brutal murder of his mother was enough disturbing satisfaction for Trinity, especially since he only chose Rita to get revenge on Dexter Morgan.