What's Achebe's concept of heroism in the novel?
Although Okonkwo has some of the characteristics of a hero (strength, power, wealth, prowess in battle, etc.), Achebe does not ultimately present him as such. Okonkwo is deeply flawed, and his weakness -- the weakness that ultimately leads to his suicide -- is his fear of the feminine. Over and over again, Achebe points out the importance of balance. Balance is a key concept in Okonkwo's culture, and is important in everything from social behavior to religion to politics. A fully realized, truly honorable man has found the balance between the masculine and the feminine. Such a man knows when to show mercy, when to go to war, when to accept change, and how to engage in empathy. Okonkwo fears becoming like his father, who was a man with no honor (and who was seen as having too much feminine energy). This fear pushes Okonkwo too far in the other direction; all of the bad decisions he makes are due to his relentless pursuit of the masculine. He is not able to bend, so he breaks others, and ultimately breaks himself. He cannot accept change, or any situation that might leave him vulnerable/not in control.
Okonkwo, as an anti-hero, creates a contrast to what Achebe ultimately presents as heroism: the balanced man.