Adrian, like Celeste, seems to be displaced from his peers and society from the start, perhaps a predisposition to feeling separate, or socially acquired, or both. He seems preoccupied with how to signify meaning, language is not enough and so he turns to music. He doesn't possess the emotional intelligence to access love or connection with others it seems, and so he turns to isolation and theology. When he does engage in a romantic encounter it is either desperate or calculated in its attitude towards self-harm. Like Celeste, this sacrifice he makes appears to be rooted in his self-destruction or surrender in giving up due to feeling displaced from the world. The loneliness and nihilistic perspective has swallowed them both and their respective attitudes move towards solipsism as a defense mechanism to avoid the world they cannot comprehend.
I don't know if it matters much if either character actually made a deal with the devil. If Celeste did, it would be of self-preservation as she is doing so for her life not for fame, while Adrian's would be for the ego in greatness, sacrificing the potential at connection that is too insecure and inspires fear, so it's the ultimate form of self-fulfilling prophecy in taking the easy route to escape from the hazy ineffable areas of life that he struggles so hard to access. The history of Germany in the book influences Adrian subconsciously towards hopelessness, much like the terrorism from school shootings to 9/11 and all in between affect Celeste's own change in attitude towards the same. Both characters are born under the umbrella of optimism, or rather humanism. Even though Zeitblom states that he grew up under a humanistic lens while Adrian did a theological one, the implication is that both boys were raised through a positive worldview that deteriorated for Adrian along with his milieu. I appreciate that his inner struggle to access his peers, or to "open himself up to others," as his friend writes, appears to be due to something inherent only exacerbated and reinforced by the worldly destruction; and I'd argue that Celeste, who also is clearly exhibiting pure humanism with her willingness to stand up and bravely try to reason with the shooter, as well as sing a song at a candlelight vigil to commemorate her peers, also sacrifices authenticity (Dafoe early on states that "back then, she used to write her own lyrics" before the shooting) and other signifiers of a hopeful identity, but this appears to be completely due to social learning theory rather than innate characteristics.
I suppose this doesn't matter either, as Adrian, who we have less insight to, may be shaped by his surroundings completely as well; we don't get that omnipotent narrator but instead his friend to tell us a perspective. In Mann's novel, the teacher who speaks of "freedom" defines it by the human being's will to choose God's path or not, but our narrator wisely presents us with the missing perspective here as one of acceptance in the spirituality of mystery as freedom too. Instead, agency as tangible action is highlighted, so Adrian of course succumbs to this reading and, by taking full responsibility and agency upon himself cannot bear to go to God so he goes to the Devil. The same could be said for Celeste, who feels no inspiration or security from spiritual mystery, as all around her is overwhelming disorder stemming from people acting on will, and so she too tries to rely on her own, a process that cannot end any way but failure. She cannot control her happiness that way, as she cannot control the music business, her sister's trysts, terrorist attacks, her lovers' faithfulness, or practically any part of her life. Without some semblance of humility and acknowledgement of powerlessness one cannot access acceptance, gratitude, or understanding. When one places the world on their own shoulders, disappointment is inevitable, and when that world doesn't match one's expectations or needs like only God or the acceptance of mystery can, well nihilism feels rather inevitable. Meaning is optimistic when it is allowed to be mysterious, but the moment is becomes a forced definition by agency that relies on the uncontrollable world molding itself to the preference of that individual, the logical response will be to declare meaninglessness by the futility of access. Anyways, so when Adrian is told that freedom is either the freedom to will oneself to unclear territory or to sin which is tangible, he chooses the easier path that corresponds to his core beliefs and sociopolitical experience. When Celeste fails to achieve happiness in the face of a threatening and horrifying world of harmful mysteries, she also chooses retreat to sin. Who wouldn't, when they only rely on themselves for support?
I think it's important to mention that I am not assigning blame to either character. Even going by recent evidence, human beings have a finite amount of will power, and studies have shown that social support and a belief in a higher power (whether god or simply internalizing acceptance that the scientific processes of the universe are outside one's control) are two of the key factors to alleviate that stress and strengthen one's will power before it depletes. So when either of these character face the world alone, it is not their 'faults' that they are unable to access a sense of connection with others to use as support, nor is it their fault that they cannot overcome the fog of their terrifying worlds to see an optimistic light. But just like Mann doesn't seem to be advocating for nihilism as much as compassionately drawing a situation where one could fall into that clouded mindset, Corbet seems to be doing the same; and just like Mann, Corbet can identify with that hopeless view enough to offer that opposite reaction of conscientiousness to empathize with the characters, whether through the best friend's narrator in the book or through our objective perspective in the film (certainly not the film's narrator, whose function seems to mirror an objective perspective without any emotion, one of apathy to match a part of Corbet, which is then complemented by the camera's distanced sympathy that mirrors our own emotional lability to the experience at hand with a chaotic score and moody shifts across a consistent tone of discomfort - that is which elicits our empathy).
If we take the narrator's second-hand information at face value, the film's final lines:
Narrator wrote:[The Devil] whispered [Celeste] melodies, and she returned with a mission to bring great change to the next century. He said, “Shut your eyes and repeat after me. One for the money, two for the show. On three we get ready. And on four, come with me.”
seem to indicate either a) a condescending portrait of pop music as the devil's work, or b) a determinist fate through which the devil offers Celeste a sealed future but provides her space to feel good within it. If we go with B (my preference), this is fitting with the complex worldview Corbet expresses in this film, and itself splits into two parts - or a range between two poles. The optimistic half of this outlook would be that we have limitations to change the bleak path we are on, and yet we can find some solace in certain spaces (art, bringing people joy, empowerment), which provides free will within determinism. On the other hand, there is potential harm in the dissonance of escapism by engaging in those acts of solace while ignoring the space that we could potentially be focusing and trying to issue change; in which case this deal with the devil resigns Celeste to accepting doom and an identity engulfed in her art, not allowing her other neglected emotional parts to get the attention they need, just as it resigns Adrian to a very similar fate. One side of the coin is the freedom toward God in Mann's book, while the other is the freedom to choose sin. We either embrace the attitude of positive surrender, that by accepting what we can and cannot control, stay right-sized and remove absolute agency from ourselves, we are afforded a freedom that allows for joy, gratitude, pleasure, positivity; or we move towards an attitude of negative surrender, of expectations, apathy, resentment, social comparing and completely giving up. If we allow ourselves relief from whether or not an actual deal with the devil matters or whether it's symbolic in each story, the same is true: it's a self-fulfilling prophecy either way.
I still think Corbet argues that both ends are on a spectrum that makeup our experience, and that if simplified, each is a part within us that voices its perspective; and I believe Mann is actually doing the same. So this "deal with the devil" in an abstract form can function as a positive or negative form of surrender, born from the same space of discomfort, distrust, fear, isolation, and trauma; but it can take the form of resilience or self-immolation as the result of recognizing one's place in an abrasive and unpredictable world. Both Corbet and Mann appear to be showing us characters that skew themselves to the default in the negative space of sin, of nihilism, and don't judge them for it or dub them weak but offer them up as examples of one way we can go, and an enticing one at that, one that feeds our protective parts and moves towards the magnet of safety in choosing our own downfall versus exposing ourselves to a scary world. The key is that the authors, not the characters, are the objective voices that level our awareness to other options. They validate this position without championing it as the only solution. Instead, through that validation they offer hope with constraints, insofar as we can perhaps acquire some kind of a balance if we try hard enough, despite the inevitable determinist factors outside of our control. Each appears to be a cautionary tale of what happens when we move to giving up completely, but if we channel that anxiety toward a space with more support, in art and people, and awareness in facing the uncomfortable truths of our world today, then we have a fighting chance at accessing some meaning in a life under the limitations and possibilities of existential agency.