Why did Okonkwo kill himself in the novel Things Fall Apart?

Okonkwo kills himself in order to deny the commissioner and his men the chance to kill him instead. After killing the commissioner's messenger, Okonkwo knows the white men will be coming for him. He has also come to understand that the people of his tribe have no wish to go to war. He has nowhere to turn, thus, he chooses death by his own hand.