I think that there are aspects to Daru's characterization that make him heroic. He does not blindly follow orders, hoping that the Arab prisoner escapes. Daru recognizes the political nature of the charges against the Arab prisoner. He seeks to make right that which is wrong in giving the prisoner a choice. He gives the prisoner money and food in making this choice as well as treating him with dignity in enabling him to choose. Outside of this, Daru shows loyalty to his students and recognizes what is right in the face of what is being told. These are all elements of his heroism.
The challenge I find in labeling him a full hero is that Daru wishes to not be involved. While he does so much that represents his loyalty and allegiance with a particular cause, Daru wishes to relinquish his responsibility of choice. He wishes to withdraw, to be left alone and isolated. On one hand, Daru recognizes his loyalty to the French, and on the other, he identifies with the prisoner. The refusal to make a choice betwee the two is where heroism is absent. Daru wishes to run away from what he knows he has to do. This desire to be relieved of duty and sense of being in the world is where he lacks heroism.
Daru sees his own role as one in which there is no choice in the matter. He thinks that he lacks a choice. It is for this reason that he leaves the choice to the prisoner, hoping that he will make the decision for Daru. Daru does have a choice in deciding the reality of his condition. He can deliver the Arab prisoner to the tribe or deliver him to the police station. He can choose. His bitterness at the end comes from the misunderstanding that he "handed over" the prisoner. While it is unfair to label him with this, Daru created this condition with his refusal to choose, to decide what he represents and believes. Daru does not see a choice, but there is a choice present. Daru can select who he is and what he believes in, and yet he does not. It is this condition that causes Daru to not see a choice, but it is one in which a choice can be made if Daru forgoes wanting to remain neutral and free from having to make a choice.