What chapter shows the dark side of Umuofia?

One aspect of darkness in Things Fall Apart is the negative influence of religion. Although Achebe lays out many negative aspects of the introduction of Christianity to Nigeria, events in the novel make it clear that continued adherence to traditional religion and governance also had a dark side.
Chapter 7 illustrates the latter aspect. In return for the earlier killing of a girl, the powerful Oracle decrees that a sacrifice is needed. This is not part of an attempt at justice against the perpetrator. It is a sacrifice of a valued, innocent member of society. The person named is Ikemefuna.
For Okonkwo, this creates a moral dilemma, as he loves this boy, who has lived with his family. He does not want to participate in the killing, and at first, his presence at the event makes him seem just complicit. Ultimately, however, he must be the person to finish the killing, even though Ikemefuna turns to him and cries ("Father!") for help.
In the traditional society, a man was considered strong if he took bold, violent actions. But being strong also meant following what the Oracle decreed. Although Okonkwo, after the killing, is tortured by his personal betrayal of the boy—a choice for which he will later pay with his own life—he understands its necessity for his place in his society.