I believe that Achebe wants the cultures responsible for the colonization of other countries to reflect on what happens when "bringing civilization" to already civilized societies.

The culture into which the "white man" intruded was already built upon strong values of family, justice, work, faith, education, and had survived many centuries without needing any help.  They had systems in places to deal with poverty, loss, failed crops, marriage, bartering for their needs and other aspects of what we all consider worthwhile in a culture. The "white man" decided it wasn't a godly culture and therefore must be destroyed. 

Achebe has the high commisioner reflect upon his godliness and actually has showed to all of us his true godlessness and blasphemy in taking on the role of a god to change what God had already deemed good. 

One of Achebe's main purposes with this novel is to show the value of traditional Nigerian culture.  He also wants to show how devastating colonialism was to the culture and lives of the Nigerians.  Okonkwo represents this culture.  For the first portion of the novel, Okonkwo is the main character and he is considered one of the most honorable by his whole village.  Then, through a mistake, he is marginalized and sent to his mother's village as punishment.  When he returns, the British culture has taken over.  By the end of the novel, Achebe switches over completely to the Commissioners thoughts showing how Okonkwo, who represented the culture, is no longer significant - the English have devastated the Nigerian cuture.  The Commissioner degrades Okonkwo's life to mere details.