Things Fall Apart by Chinua Achebe ends tragically as the whole story is tragic. The main character of the story, Okonkwo, has a tragic history. He has bitter memories of his father. He thought his father a weak man. His father, Unoka, died leaving many village debts unsettled. Okonkwo has to live with that. He is a fierce warrior and takes pride in this fact.
Since Okonkwo thinks his father was such a failure and died in disgrace, he is determined to not follow in his footsteps. At the end of the book, you see that Okonkwo's own choices in his life have led to disgrace for him and his family, as well. The one thing he didn't want to happen, has now fallen onto him. Okonkwo's death, by suicide, is considered a grave sin, he has brought shame to himself and his family. Making the choices he did in his life, Okonkwo unknowingly has ended his life in the same disgrace as his father did.
The whole story is about choices. If Okonkwo had made different choices and listened to what other people had told him, would he have ended up the same way? By taking his own life, he has made his family and children have to live with the same amount of shame and disgrace as his father did. Did he stop and think about that as he was taking his own life? The author really sets up difficult questions for us, as readers, have to ask ourselves.