Ikemefuna had been placed in Okonkwo's care after it had been decided that another village, Mbaino, had to offer up a young virgin and a young boy as compensation for the killing of Ogbuefi Ujo's wife when she went to market there. Ikemefuna, who was fifteen at the time, lived in Okonkwo's care for three years before it was decided that he had to be executed.
Ezeudu had asked Okonkwo not to be involved in Ikemefuna's execution, but when the men came to take Ikemefuna, Okonkwo accompanied them. When they were far outside of the village, one of the men struck Ikemefuna, but the blow did not kill him. Ikemefuna cried out, "My father, they have killed me!" and Okonkwo, out of fear, struck him down. He was afraid that the men would think him a coward if he did not do so.
This fear is born out of Okonkwo's pride. He always had to prove himself a man and his harsh actions towards his family and others spoke volumes of what it meant for Okonkwo to be a man.
It is in this sense then, that Ikemefuna's killing foreshadows Okonkwo's fall, for it was this same fear, borne out of pride, that led to Okonkwo killing the court messenger who came to break up a meeting in their village.
The gathering had been called to discuss what kind of action was to be taken against the evil which had invaded their society - this is, of course, a reference to the missionaries. Okonkwo had been a the forefront of protest and they had burned down Enoch's compound and the church. The village had to pay 250 bags of cowries for their men, Okonkwo included, to be released from captivity.
After the messenger's killing, the Commissioner and a band of soldiers came to arrest Okonwo.
Here again, it was Okonkwo's pride which drove him to suicide. Instead of succumbing to the will of the Commissioner and his men, Okonkwo hanged himself, in spite of his action being deemed an abomination by his clan, who would refuse to bury his body or even touch it.