This is an interview Achebe did with PBS in May 2008. Once you get to about the 3:10 mark, Achebe explains why he didn't paint a completely idealized version of Ibo culture and why Okonkwo is so flawed in Things Fall Apart. He says "I knew that I wanted the story to be true in the way fiction can be true." I think the fact that Achebe showed Ibo culture as flawed, in a way establishes his credibility. Meaning, we, as readers, can be more confident in knowing that Achebe is painting us an accurate picture of Ibo culture.