When I read the final two chapters of Things Fall Apart, the nature of Okonkwo's death struck me as heroic and redeeming. I read his suicide as an attempt to save his village from retribution while keeping his honor, as well as an act of desperation and frustration. Alone among the people of Umuofia, Okonkwo has the courage, the foresight (he sees that his culture cannot coexist peacefully with the colonizers), and the desire to save his culture to stand up to the District Commissioner. Killing the court messenger is the only way he can keep his pride and his commitment to opposing European encroachments on his village. It is a violent and ultimately futile action, but he has no good options by that point--he can't simply let the messengers break up the meeting after promising to fight to keep Umuofia's independence.
I think that by portraying Okonkwo's action as futile in spite of his overwhelming strength, determination, and courage, Achebe is trying to convey the frustration and despair of seeing one's culture slipping inexorably away.