Because Okonkwo have to kill him because the Oracle decreed that Ikemefuna must die:)
The Oracle of the Hills and the Caves pronounced that Ikemefuna must die, so they meekly followed their gods' orders.
He said that Okonkwo shouldn't have went with the men to kill Ikemefuna and that he will be punished by the earth goddess for what he has done.
Okonkwo struck the last blow to Ikemefuna because he did not want to seem weak or cowardly, even though Okonkwo likes Ikemefuna, and thinks of him as his adopted son.
The village decides to kill Ikemefuna because the Oracle of the Hills and Caves decrees that he must be sacrificed. The villagers do so to avoid bringing wrath upon themselves and their village. It is a tragic decision made out of fear and obedience to their spiritual beliefs.
Obierika is saddened and critical of the village's decision to kill Ikemefuna, as he viewed him as an adopted son and struggled to come to terms with his death. He questions the wisdom of the oracle and the customs that demand such sacrifices.
Okonkwo struck the last blow to Ikemefuna because he did not want to seem weak or cowardly, even though Okonkwo likes Ikemefuna, and thinks of him as his adopted son.
Ikemefuna is led by the tribe on a long march outside of its lands. One tribeman cuts him with a machete. As Ikemefuna runs to Okonkwo for safety, Okonkwo chops Ikemefuna down with his machete.
Okonkwo becomes a father to Ikemefuna, and Ikemefuna calls Okonkwo father. Ikemefuna is like the second man in the house, the closest to Okonkwo's personality, and helps to develop Nwoye's manhood. Okonkwo likes Ikemefuna, but does not show his feelings towards Ikemefuna because to do so would be unmanly.
Mbaino is the village which Ikemefuna lives in before he is forced to live with Okonkwo in Umuofia in order to repent his father's crime. It is significant because in essence, things begin to "fall apart" for Okonkwo after Ikemefuna is killed by Umuofia. Okonkwo becomes haunted by his choice to kill Ikemefuna. In fact, Ikemefuna's death begins Okonkwo's downfall.
Ikemefuna came from Mbaino.
Ikemefuna called Okonkwo father. It would be unnatural for a father to kill his son. The boy was also affectionate towards Okonkwo, and it would be like a betrayal.