Because the beginning of No Longer At Ease has already provided us with the knowledge that Obi Okonkwo is guilty of corruption and accepting bribes, the actual ending of the novel is ironic. Achebe chooses not to belabor or repeat the scene in the courtroom where we learn of Obi's guilt. Instead, he ends with just a few sentences explaining that nobody knows why Obi began to accept bribes, or how he became corrupt.
We are left with a number of unsettling questions. How could Obi's family not know about his corruption? How could his parents, who willingly accepted Obi's monthly monetary gifts, not know they were part of the problem? How could his mother not know that threatening to kill herself would somehow affect him? How could the Umuofia Progressive Union not know that Obi's repayment of his debt would sometimes be difficult, more difficult than he could bear? How could they not see the stresses and strains Obi endured as he tried to become part of a class of society that he knew nothing about and could not understand? And given that bribery permeated their community, why would they assume their son to be immune?
Throughout the novel, we observe that the other people in Obi's life are complicit in his downfall. It's true that his corruption is ultimately due to his own hubris, but it's also clear that all the different people in his life play a role – from his relatives who wouldn't let him love the girl of his choice and who taxed him heavily in order to pay back his debt, to his peers in the government who demanded that he conform to a particular social lifestyle even though he couldn't financially afford it, to cultural expectations that he help his little brother through school and support his parents in their old age.
Even if everybody else pretends they're ignorant, we know the truth. They were all part of the reason why Obi became corrupt. If the beginning of the novel leaves us curious about how Obi becomes a corrupt politician, the ending points out that it is not an easy answer, possibly suggesting that our individual actions are heavily influenced by the situations we find ourselves in, the people who surround us, and the choices that others make, as well as our own.