One quote that hints at Okonkwo's downfall is seen in chapter 13. In this chapter, Okonkwo hears that Ezedu is dead. He reflects on the last conversation that they had before the respected man died:
A cold shiver ran down Okonkwo's back as he remembered the last time the old man had visited him. "That boy calls you father," he had said. "Bear no hand in his death."
Ezedu was a highly respected warrior in Okonkwo's community; still, Okonkwo acts against his advice:
Ezeudu had taken three titles in his life. It was a rare achievement. There were only four titles in the clan, and only one or two men in any generation ever achieved the fourth and highest. When they did, they became the lords of the land.
Ezedu's reference to "that boy" concerns Ikemefuna. Ikemefuna is a fifteen-year-old boy who is taken (along with a young virgin girl) from a neighboring clan. This clan gets involved in a conflict with Umuofia (Okonkwo's clan). Okonkwo is sent to Mbaino (the other clan) as an "emissary of war" (chapter 2). The other clan offers these two children to Okonkwo to maintain peace between the clans.
And so for three years Ikemefuna lived in Okonkwo's household (chapter 2).
While living with Okonkwo, Ikemefuna starts to be treated like part of his family. For most of this time, the conflict with the neighboring tribe is entirely forgotten:
For three years Ikemefuna lived in Okonkwo's household and the elders of Umuofia seemed to have forgotten about him. . . . He had become wholly absorbed into his new family (chapter 7).
Later, some of the town leaders come to speak privately with Okonkwo. After they leave,
Okonkwo sat still for a very long time supporting his chin in his palms. Later in the day, he called to Ikemefuna and told him that he was to be taken home the next day (chapter 7).
Okonkwo's body language suggests that something is upsetting him. We soon find out that the men have decided to kill Ikemefuna as an act of justice against the neighboring tribe. The men set out together (supposedly to take Ikemefuna back to his home). Eventually, Ikemefuna is told to keep walking forward and not to look back. He notices that Okonkwo moves to the back of the group, and Ikemefuna grows afraid. A man then raises his machete against Ikemefuna:
As the man who had cleared his throat drew up and raised his machete, Okonkwo looked away. He heard the blow. The pot fell and broke in the sand. He heard Ikemefuna cry, "My father, they have killed me!" as he ran towards him. Dazed with fear, Okonkwo drew his machete and cut him down. He was afraid of being thought weak.
In this scene, Okonkwo disregards Ezedu's advice. He participates in the killing of Ikemefuna (who is like a son to him). This murder (and his refusal to listen to Ezedu's wisdom) hints at Okonkwo's coming downfall.