Why does Okonkwo kill Ikemefuna in Things Fall Apart?
Even though Okonkwo loves Ikemefuna and in some ways thinks more highly of him than of his own son, Okonkwo participates in the death of Ikemefuna because it has been decreed by the oracle, and he is also scared of being thought to be weak by the other men with him. One of the major aspects of Okonkwo's character is the way that he set himself to be completely different from his father, who was thought to be weak, feminine and not manly enough. Therefore, throughout the novel, Okonkwo deliberately pushes himself to show everybody, including himself, how manly he can be. Note how the text describes the death of Ikemefuna in Chapter VII:
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.
What drives Okonkwo to participate in the killing of Ikemefuna therefore is his fear of being perceived as "weak" if he hung back and did not participate. Even though others have counselled him not to be involved in this killing, Okonkwo's fear of what others might think of him and what they might say drives him to kill Ikemefuna, which is an action that will haunt him for some time afterwards.
Okonkwo kills Ikemefuna even though Ikemefuna viewed Okonkwo as a father because Okonkwo does not want to appear weak in front of the villagers. Okonkwo does not want to be judged as one who has suddenly become weak and affectionate because of a boy. To prove the villager that he is still strong, he decides to take part to kill Ikemefuna.
Okwonkwo didn't want to seem weak, and he was a traditional man. He believed ikemefuna's death was was something that had to be done, therefore he separated his feelings and his "responsibility".
When Ikemafuna was stabbed in the back by a machete, he yelled out to Okonkwo to save him but in fear of looking weak (which stems from his issues with his father) in front of his friends he finished off Ikemafuna.