Daru is a man who deeply values his freedom and would not impose on anyone else's even when he is told to by the army. He is a solitary man who would rather not be faced with the questions and responsibility of someone else's imprisonment or freedom. He does not say so, but he believes in the equality of men which is reflected in his actions throughout the story. He treats the Arab prisoner as an equal or a guest by feeding him and making sure he is comfortable.
Balducci brings the Arab prisoner to Daru with orders that Daru is to take the Arab prisoner to the prison in Tinguit. He tells Daru that the Arab prisoner is accused of murder and seems unconcerned whether the Arab prisoner is guilty or innocent. He does the bare minimum to make sure the orders are carried out, yet he is insulted by Daru's attitude.
The Arab prisoner
The Arab prisoner drinks all the tea Daru serves him when Balducci is in the schoolhouse. He also eats the food Daru serves him twice and does not try to escape. This belies the crime he is accused of which is murdering his cousin. He does not seem temperamental but seems lost and afraid.