"Dead Men's Path" is a story about the conflict between tradition and modernity in Africa, particularly in matters of religion. A good thesis statement will bring out the details and subtleties of this conflict, in which the author appears to have some sympathy for both sides. Michael Obi's downfall comes not because he is malicious or fundamentally wrong, but because he is unwilling to compromise. The village priest defending the ancient superstition comes to appear a more reasonable figure, precisely because he advocates a position of mutual tolerance and respect, rather than complete victory for his own side. You might use the priest's own words for a thesis statement expressing this point:
Chinua Achebe's solution to the conflict in the story is expressed in the words "let the hawk perch and let the eagle perch."
You might use this statement as a basis to explore the ways in which Michael Obi goes too far with his reforms. For instance, he tells the priest that it is his duty, and that of the school, "to teach your children to laugh at such ideas" as the dead men's path. If he had said that he wanted the children to question these ideas, this might have been reasonable. However, it is inflammatory and counterproductive to say that he wants the children to laugh at ideas which are important to their parents and in their culture and which have done no harm.