In Arrow of God, both the British Captain Winterbottom and the Igbo Ezeulu have inflated senses of duty, which might be why the two men like each other. Winterbottom believes it is his duty to maintain decorum, keep a high moral standard, be an example to others, and be obedient to the Administration's whims even when he doesn't agree. Ezeulu, alternatively, believes that he must do whatever the god Ulu requires of him, even when it's distasteful, and even when he personally suffers as a result.
Because Winterbottom and Clarke have an inflated sense of duty to Great Britain, they are able to justify jailing Ezeulu for his refusal to be warrant chief in Umuaro.
Because Ezeulu has an inflated sense of duty to Ulu, he is able to justify harming his people by refusing to declare the Feast of the New Yam in time for them to harvest their yams and prevent famine.