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.
Unfortunately, Okonkwo played quite a major role in the death of Ikemefuna. Let us remember that since he was taken to become part of Okonkwo's family, Okonkwo had become incredibly fond of Ikemefuna, even preferring him in some ways to his own son, Nwoye. He certainly has a positive effect on Nwoye's development that Okonkwo is definitely not blind to. However, when the news comes that Umuofia has decided that Ikemefuna must be killed, in spite of warnings not to have anything to do with his death, Okonkwo insists on showing his manly nature by going with Ikemefuna to his slaughter and even being involved in the act himself:
As the man who had cleared his throat drew up and raised his matchet, 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 matchet and cut him down. He was afraid of being thought weak.
Note how the fear of what others think of him impels him to cut Ikemefuna down, even though he calls Okonkwo his "father." Okonkwo's determination to be viewed as a strong man in his tribe causes him to act in a way that even he recognises is wrong at some levels, as his fasting following the act shows.