Ikemefuna's fate is one of the most difficult parts of Achebe's novel. It's not much of a mystery that Ikemefuna will come to a bad end: he's introduced as the "doomed lad" who was given to Umuofia explicitly as a sacrifice. Okonkwo's role in Ikemefuna's death, however, is not clear, but is foreshadowed by the narrator's assessment of Okonkwo's character in the paragraphs immediately following Ikemefuna's coming to live with Okonkwo. Okonkwo hates everything his father loved, especially "gentleness and idleness." It's clear that Okonkwo's fear of being seen as "weak" will cause him to act against the boy, and, for the boy's part, "he could not understand what was happening to him or what he had done."
Okonkwo's growing regard for Ikemefuna, Ikmefuna's friendship with Okonkwo's son Nwoye, and Ikemefuna's tendency to refer to Okonkwo as "father" all point to a reckoning, since we know that Okonkwo eventually will not be able to abide by the tenderness he feels for the boy. In fact, it is because Okonkwo has become the boy's father figure that he is told to not participate in his killing, but it is this prohibition that naturally causes Okonkwo to kill him as a show of strength.