You have put your finger one a very important aspect of Michael Obi's characterization here. When he begins to get the school back in shape we, as readers, can see that Michael Obi values both ambition and idealism.
We know Michael Obi values both ambition and idealism because of his desire to reform a decrepit old school. Ambition in itself isn't always bad. It is only when it takes the route of the "vaulting ambition" of Macbeth that it becomes evil. With Michael Obi we see a tendency towards this type of ambition with his thought that "only he" can save the school. Michael Obi speaks of the former practices at the school with disgust.
[Before me it had] the narrow views ... and superannuated people in the teaching field.
On the positive side, Michael Obi vows to institute contemporary methods and very lofty standards. Both of these qualities show Michael Obi's idealism. Michael Obi, then, idealistically thinks his dreams of European standards easily overshadow the natives' traditional belief system.
In conclusion, and as you might already suspect, the problem with Michael Obi is what he does not value: tradition and belief. We find out at the end of the story that the villagers, knowing their traditions and beliefs are not accepted, destroy Michael Obi's chances at success.