It is never specified exactly why the Oracle has called for Ikemefuna to be killed. We only learn that the Oracle has called for his killing as part of a ritual. The men in the tribe either don't question the Oracle, or they do not share the reason with Okonkwo. Okonkwo is only told not to participate, because Ikemefuna calls him father, and he is like a father to the boy. Part of the theme of this novel is that the Ibo tribe often does things without knowing why, it is just tradition, just ritual. The killing of Ikemefuna is a tragic illustration of how the tribe has these beliefs and rituals that are often harmful and illogical. One part of the ritual is that a father is not supposed to participate in the ritual killing of his son, even if it is ordered by the Oracle, and Okonkwo goes against this because he does not want to appear weak in front of the other men. Bad decision.
Okonkwo's life falls apart after this because he has taken part in the killing of his adopted son. His real son Nwoye becomes estranged from Okonkwo at this point, never to return, because he loved Ikemefuna as a brother. The tribe itself begins to fall apart after this as well. The younger people begin to question some of the tribe's beliefs (like Nwoye) and begin to rebel against blindly following things that don't appear to make sense to them. This eventually leads Nwoye to accept the Christian religion.