There are a number of very important quotes in this play that could be discussed in response to this question. One important quote comes in Scene Two, and occurs when Pilkings, talking about the natives, refers to them as "Sly, devious bastards." This is made even worse by the way that he says this in front of his house boy, Joseph, who is one of those "natives" he is refering to. This quote speaks volumes of the arrogance of the whites in this play, and this is something that is supported in Scene 4 when Olunde makes the following observation about whites:
You believe that everything which appears to make sense was learnt from you.
This indicates the particular point of view of the white characters in this play. They believe that they are more civilised, refined and cultured than the "ignorant natives" they are ruling over, and that their way of viewing the world is the only correct way there is. This play of course challenges such cultural arrogance by insisting each culture has their own valid world view.
Secondly, as regards the theme of duty, Elesin's comment in Scene 5 to his young bride is worthy of some attention. He explains that he failed to commit suicide not because he was interrupted by the colonial forces, but because he loved life too much:
For I confess to you, daughter, my weakness came not merely from the abomination of the white man who came violently into my fading presence, there was also a weight of longing on my earth-held limbs.
This is important because it presents Elesin's failure to complete his obligations as resulting from his own personal longing to live life more rather than it being the fault of the whites. This presents the theme of duty and honour in very stark terms, as Elesin himself acknowledges that he loved life too much to end it, even though this has had catastrophic results for his people.