In Things Fall Apart, Achebe utilizes an omniscient narrator who speaks in the third person. This omniscient narrator often dips in and out of different characters' perspectives, providing multiple different points of view on a particular situation. When the narrator dips into one specific character's perspective, the tone of the novel will shift to suit that character.
One such shift occurs in Chapter 25, during which the narrator adopts an ironic/satirical tone to explore the District Commissioner's character:
As he walked back to the court he thought about that book. Every day brought him some new material. The story of this man who had killed a messenger and hanged himself would make interesting reading. One could almost write a whole chapter on him. Perhaps not a whole chapter but a reasonable paragraph, at any rate.
It is worth noting that, up until this point in the text, Achebe uses satirical tonality sparingly. Consequently, when the tone shifts in this passage towards greater satire and irony, the change is noticeable to readers. The District Commander's thought process is abhorrent. Achebe wishes to call attention to this and invite readers to scrutinize the Commander's behavior—thus, the introduction of a noteworthy tonal shift.