Having the devil inside you will change you, at least that's what happened to Nick on Chilling Adventures of Sabrina. At the end of part 2, Nick (Gavin Leatherwood) absorbed Lucifer, aka the Dark Lord aka Sabrina's dad (aka Luke Cook), into himself to trap the devil and keep the mortal realm safe. Like a Satanic tupperware container. But, understandably, he almost instantly regretted taking that responsibility.
What's Wrong With Nick On CAOS?
At the start of part 3, Sabrina (Kiernan Shipka) rescued Nick from where he was trapped in Hell and she figured out a way to transfer Lucifer to Father Blackwood's (Richard Coyle) body. Nick was free — or so they thought. Having Lucifer's soul trapped inside him left its mark on Nick. Even after his spirit was expelled, Nick was convinced that not all of him was gone. Sabrina brushed off Nick's feelings, telling him it was just PTSD. However, Lucifer said the opposite in episode 4, telling Nick, "They'll always be a piece of me inside you." It turns out that Lucifer was being literal, because in episode 5, Ambrose (Chance Perdomo) and Sabrina discovered that a black, gooey substance remained in Nick's body. It was remnants of the devil, which Ambrose likened to "tar in a smoker's lung … [except] alive, infernal, and corrupting."
That part drove Nick to drugs and alcohol to cope with the trauma of having been locked in Hell fighting a daily battle with the devil inside himself. He cheated on Sabrina, turned his back on those who needed him, he was not himself, and that's simply because he wasn't all himself. For a while, Nick was also part-devil still. In the fifth episode, Ambrose used Sabrina's blood to draw the black goo out of Nick. It was baited by the devil's DNA in her blood since Sabrina is Lucifer's daughter. Then it seemed like Nick was truly free.
Do Nick & Sabrina Get Back Together?
But expelling the demon within him didn't change how Nick felt. He broke up with Sabrina because he was just too scarred by the whole experience and too reminded of it whenever he saw her. Nick was concerned that the way he acted may not have just been the Dark Lord controlling him, but rather harnessing a part of himself that may have already been there. "[What if he] activated, exploited something that was already there? Some need? Some weakness?" Nick worried.
Sabrina tried to allay his fears, saying that, now that the goo was gone, Nick really was back to just himself. But Nick walked away from their relationship anyway. At the end of part 3 he sat with Prudence (Tati Gabrielle) on the steps of the school and confessed that he felt alone. The evil within him is gone, but it left behind an empty feeling.
Leatherwood previously hinted at Nick's CAOS Part 3 changes, telling TV Guide that "having Satan inside you definitely takes a toll on you … mentally and emotionally." He said he planned to do a lot of acting work to figure out how to portray the new Nick, and how he's changed after such a traumatic experience. He added that while Sabrina became "more gung-ho for Nick" after he sacrificed himself to imprison the Dark Lord, the incident left Nick feeling more "resistant to it or hesitant" about his future with Sabrina.
Even though Nick has his doubts about his true character in the wake of hosting the Dark Lord, Leatherwood doesn't have those same concerns, especially after watching Nick sacrifice himself for the good of everyone by imprisoning Lucifer in part 2. "I think it shows that Nick does have a lot of good in his character, even though he was a little manipulated himself," Leatherwood said in the same interview.
Perhaps in part 4, Sabrina will help him see that's he's not alone like he thinks and that it was just the Dark Lord controlling his behavior. Even in their breakup scene, it was clear he didn't like hurting Sabrina and didn't really want to leave her. He just thought it was best given the situation and how much had unfolded. There's hope for Nick yet, he just has to find it within himself and believe that there is goodness there.