Let us remember that in Aristotle's creation of the term a tragic hero, he said that a tragic hero was a character who was basically a good person, but who had one tragic flaw that undermined their goodness and led them down the path to tragedy. If we apply this idea to Okonkwo in this brilliant novel, then we can see that the major flaw of Okonkwo is the fear he has of being associated in any way with his father, the lazy and profligate Unoka. Note what we are told about Okonkwo's character in Chapter Three of this book:
But in spite of these disadvantages, he had begun even in his father's lifetime to lay the foundations of a prosperous future. It was slow and painful. But he threw himself into it like one possessed. And indeed he was possessed by the fear of his father's contemptible life and shameful death.
Okonkwo is so concerned about doing everything he possibly can to be seen as a man that he commits actions that he really should not have committed, such as participating in the death of Ikemefuna. It is his overriding desire to be seen as a man and his determination to never let others consider him weak that results in his brusque and thoughtless acts of violence that result in his exile and again, finally, in his suicide. He is a tragic hero whose character flaw is responsible for his tragic final downfall.