Okonkwo's final decision - is it act of courage or act of protest? What meaning do you think Achebe intended us to take away?
Okonkwo's suicide is an act driven by despair. This despair seems to come directly from Okonkwo's sense that his village culture/identity has disappeared.
In ways that are quite real and deeply disturbing for Okonkwo, Umuofia has become a place (and home to a people) that Okonkwo no longer recognizes. When his village loses coherence, Okonkwo inevitably loses his own sense of self as well. Thus his suicide is perhaps not a protest and not an act of strength, but is instead a graphic admission to the fact that the synergistic relationship between individual and society has been destroyed. One no longer supports the other.
At the end of Chapter 21, Okonkwo is shown mourning "for the warlike men of Umuofia, who had so unaccountably become soft like women." We should not take this as merely a comment on Okonkwo's macho attitude. In this moment we see that the things Okonkwo is most proud of about himself -- his manliness, his prowess in battle, etc. -- are no longer of any consequence or value in Umuofia. The village has changed in ways that challenge the basis of his identity.
For a person so dependent on his culture for his own identity, the dissolution of the village identity leads to Okonkwo's own dissolution. To some extent we can see Okonkwo's inability to adapt to the changes wrought by the arrival of the Europeans as a sign of weakness. However, Obierika proclaims that Okonkwo is a victim of the colonizing mission and its agents.
"That man was one of the greatest men in Umuofia. You drove him to kill himself, and now he will be buried like a dog..."
In this outburst, Obierika suggests a depth of meaning surrounding Okonkwo's death. Okonkwo will be buried like a dog because he has committed a taboo act, signifying a break with the rules and codes of the village culture.
As a man dedicated to upholding his tribal customs (as we see time and again in the novel), Okonkwo's decision to commit an act that his tribesmen will see as weak seems to demonstrate the idea that he feels his culture is no longer intact. He cannot take strength as he once did from the cohesive, intricate and predictable customs of his village. He can take no strength from his village identity and so commits an act that finally and irrevocably divorces him from his people and its traditions.
The suicide then is a symbolic act, signifying the break that has taken place. Suicide sunders him from his people so that they cannot touch him to bury his body. A man once truly integrated into his society has lost his place among his people (and lost his sense of self) because his culture fall apart around him.