He can't. Who can justify suicide and self-destruction? It is irrational, a dark empty feeling. It has nothing to do with reason. Camus said suicide is the rejection of freedom. It is absurd.
For Okonkwo, suicide is the refusal to make a choice. It is the unwillingness to withstand a life of suffering. As such, it is unholy and shameful.
Enotes has wonderful commentary on this subject:
Okonkwo may be a tragic hero because as he stands for his convictions, his individualism results in disaster. He realizes that his efforts to save the traditional world are futile. His suicide saves him imprisonment, cheats the whites of revenge, and makes a mockery of the values of the clan. Is Okonkwo mad? What does he really stand for and what is the significance of his destruction? Does Okonkwo represent the suicidal fragmentation of Igbo society? Okonkwo’s life is ruled by fear of failure. Yet, Okonkwo fails. He is unable to understand his father or son; he is unable to balance the male and female energies in the traditional world order, and he is unable to adapt to the changes introduced by the white colonizer. Okonkwo’s suicide is shocking and ambiguous; ironically, his death is as shameful as his father’s. His friend Obierika again provides invaluable insight and a reliable picture of the collapse of traditional life. He understands that Christianity has put a knife on the things that held the Igbo together. In his bitter epitaph for Okonkwo he addresses the District Commissioner saying, “That man was one of the greatest men in Umuofia. You drove him to kill himself; and now, he will be buried like a dog . . . ” (p. 147)
Look at Obierka's words. I think he speaks for Achebe here. Obierka knows that Okonkwo's death is symbolic of tribal culture in Africa: it is not so much a suicide or self-destruction but a kind of assisted suicide. The white man gave Okonkwo just enough rope to hang himself.