In the novel, "Things Fall Apart" by Chinua Achebe, Ikemefuna becomes a member of Okonkwo's family as part of a settlement from a neighboring village that has wronged Okonkwo's clan. Over time, Okonwo begins to consider the 15 year-old Ikemefuna to be a beloved adopted son and his own biological son, Nwoye, becomes very attached to Ikemefuna.
Approximately three years later, Okonkwo's village experiences a rare attack of locusts. The village elders decide that Ikemefuna must be sacrificed to appease the situation.
Okonkwo is advised not to attend the ceremony in which Ikemefuna will be killed, but he attends anyway, despite the warnings. When Ikemefuna notices one of the men wielding a machete towards him, he runs to Okonkwo for help.
In a heartbreaking move, but true to Okonkwo's character, he delivers the death blow himself, violently killing Ikemefuna so that he does not appear weak in the eyes of the men in attendance and the village.
So, Okonkwo is Ikemefuna's killer; this is his role in Ikemefuna's death.