To what extent is Okonkwo killing Ikemefuna justified?

The killing itself is unjustifiable, but I don't believe your question is meant to clarify the Igbo's culture and tribal law. Okonkwo's part in Ikemefuna, however, is unjustifiable and unforgivable.... particularly as the boy looked on him as a father and was accepted as a member of the family. Okonkwo was purposely told to stay home and take no part in the boy's death by the village elders, as killing a "family" member was a great offense. Okonkwo, however, always worried about pride and appearances first, chose to take part.