The central theme seems to be the question of whether what people do when they are put into conditions of extreme distress represents "what they really are", or is simply a product of their environment. The modus operandi of this afterlife hinges on "drawing out darkness" by inducing fear, anger, pain, and trauma in people, and then using their actions in such a state as justification for punishment. This is all brought to a head in the climax, where Chiyuki knows intellectually and morally that pressing the button is wrong, and doesn't want to take someone's life, but is overwhelmed by emotion and begs Decim to press it anyways. It is later shown that Decim sends her for reincarnation, showing that he has allowed himself to soften, and sympathize with, Chiyuki and humanity as a whole.

Philosophically speaking, this concept is known as akrasia, the idea that one can choose something that goes against their conscious will and better judgment. In psychology, it is acknowledged that people often act in irrational, harmful, or even self-destructive ways during or after periods of profound distress or trauma. However, this can be treated. Individuals who are given help and taught coping skills can overcome or resist the effects of adverse conditions, and make decisions more in line with their beliefs and values.

There's also an underlying existentialist theme, with arbiters being essential creatures, contrasted with humans as existential ones. Arbiters do not get to choose their nature, it is determined for them at creation: to judge. Even if they could choose to change themselves, as Decim seems to, it's ultimately futile, as their memories are wiped every several months. After only a short time, all that will remain of Chiyuki is that dummy, and he will return to applying the same draconian standards of cruelty as he did at the beginning. Arbiters are eternal, both immortal and unchanging. By contrast, humans are emotional, temporary, and capable of choosing, or even changing. Interestingly, the other denizens of the afterlife, such as Nona, Castra, and Oculus, are immortal as well but have existential freedom, allowing them to choose, or perhaps, even change. Perhaps, the state of things, of trauma, condemnation, and damnation, might someday end. Perhaps not all is written in stone, and there may someday be a better world.