Prequels are often interesting because of the ways in which they shed new light on the events of the main series to which they are a prequel. To this end, The Kill Order is interesting because it provides new insight into what it means to be infected with the Flare. Namely, the structure of The Kill Order's narrative gives us inside into the inner life of Mark as the Flare drives him slowly insane. In this regard, the novel joins a rich and nuanced tradition of literature with various forms of unreliable narration. This entry catalogues some modes of unreliable narration in order to better situate to kind of unreliability that manifests in The Kill Order.
Crucial in understanding forms of unreliable narration is the distinction between the focal character of a text and the narrator of the text. The focal character is the character in a text on which the narrative is focused: the narrative may be conveyed through this character's perspective, or the narrative may just describe events that happen to this character, albeit not from that character's perspective. The narrator, on the other hand, is the entity that conveys the narrative to the reader. This entity may or may not be a character within the text; if the narrator is a character in the text, then the narrator is typically the focal character as well, conveying the narrative through his own perspective; if the narrator is not a character within the text, then the narrator often has some degree of omniscience, or a "god's-eye view": he has knowledge of all events in the narrative and is able to focus on various characters to various degrees.
If a narrative is unreliable, the sort of focal character and narrator in that narrative will determine the kind of unreliability that it exhibits. Consider, for instance, a situation in which the narrator is also the focal character, and that narrator is unreliable: in such a case, because the entire narrator is responsible for conveying the narrative to the reader through his own perspective, the status of the whole narrative's truth can be thrown into question. If the narrator himself cannot be trusted, then how can we be sure that the narrative he is telling us reflects what "really happened"? As you might imagine, such narratives threaten to be very confusing and difficult to interpret because they are colored by global unreliability. Some famous examples of this sort of unreliability include William Faulkner's The Sound and the Fury and Henry James' The Turn of the Screw. (Notably, this kind of global unreliability often makes it a controversial matter whether a given narrative even is unreliable or not.)
Now consider in contrast a case in which the narrator is omniscient, and the focal character is unreliable. In fact, this is just the sort of case that we have at hand in The Kill Order: Mark, the focal character, becomes increasingly unreliable as the Flare's insanity takes hold of him. Because the narrator exists outside of the narrative and the focal character's perspective, we as readers are able to trust in the veracity of the overall narrative, while still being shown glimpses into the decaying mental life of Mark, the focal character. Contrasting this sort of local unreliability with the global unreliability discussed above, we can see why this narrative structure would work especially well for the purpose of this novel in relation to the Maze Runner trilogy. It seems safe to say that one of the novel's main goals was to give readers insight into more of the world and history that led to WICKED, and to the events of the main trilogy. In this regard, it is crucial that the prequel provided a reliable overall narrative, such that the reader could be confident they had actually learned an accurate history of the Maze Runner world, while also detailing the insanity-inducing effects of the Flare. This is just what local unreliability is able to achieve for the narrative; understanding this underlying literary structure allows us to appreciate just how the narrative achieves its intended effects.