One of the major weaknesses in Okonkwo is his belief that personal agency is stronger than almost any other force, even that of the gods. He constantly recites the proverb that "when a man says yes his chi says yes also." This belief causes him to ignore any sense of responsibility to his community and even to his family. He demonstrates this by breaking the peace during peace week and beating one of his wives. He also demonstrated it by ignoring the advice of respected members of the clan repeatedly, including when he kills Ikemefuna.
Because of this, he is unable to understand or value the idea of social responsibility or accountability. It gets him banished to Mbaino but even this severe punishment fails to change his outlook. He tries desperately to bend the clan's will to his own when he tries to raise up an attack on the white men. Even when he realizes that this effort has failed, he cannot adjust and change his plan. He is so eager to pursue his own agency that he fails to see the fact that the clan will pay for his rash decisions.
His inability to understand his responsibility to the clan not only hurts the clan in the long run but contributes to his own downfall.