Because Okonkwo have to kill him because the Oracle decreed that Ikemefuna must die:)
The Oracle of Agbala pronounced that Ikemefuna must die.
Ikemefuna is taken from his village as part of a settlement for the death of a Umuofian woman in the marketplace of his village. He goes to live with Okonkwo. Later, Ikemefuna is sentenced to die, but he does not know it.
The Oracle of the Hills and the Caves pronounced that Ikemefuna must die, so they meekly followed their gods' orders.
The Oracle of the Hills and the Caves decreed that Ikemefuna should be sacrificed as compensation for the murder of a woman from Umuofia. This decision was final and carried out by the villagers, including Okonkwo, despite Ikemefuna's close bond with the family.
Ikemefuna is taken from his village as part of a settlement for the death of a Umuofian woman in the marketplace of his village. He goes to live with Okonkwo. Later, Ikemefuna is sentenced to die, but he does not know it.
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.
Ikemefuna came from Mbaino.
The village elders told Okonkwo to take care of Ikemefuna until his fate was decided. When Ikemefuna is sentenced to die, Okonkwo is afraid of being seen as weak. When Ikemefuna cries to him, Okonkwo takes care of Ikemefuna, (kills him).
Ikemefuna is taken into Okonkwo's home as part of the compensation for a murder committed by the neighboring village. He lives with Okonkwo's family and forms a close bond with Okonkwo's son, Nwoye. However, tragically, Okonkwo ultimately participates in Ikemefuna's killing to demonstrate his masculinity and avoid appearing weak.
Although Ikemefuna is from a different family, Nwoye treats Ikemefuna as an older brother to look up to and mimic. Ikemefuna is the older brother that Nwoye never had.
Okonkwo is part of the group which leads Ikemefuna out of the village to be killed. After Ikemefuna receives the first blow and runs towards Okonkwo for protection, Okonkwo strikes Ikemefuna down, not wanting to be seen as weak.