The answer is not an easy one, as it lies somewhere in between the call of duty and the pull of free will.
Duty, in the sense of social expectations, but also in kindness towards a fellow human being, compells Daru. As the two come to the literal peak of decision (they have reaced the "top peak" of a mountain and look down upon a plateau): "Daru surveyed the two directions. There was nothing but the sky on the horizon. Not a man could be seen. He turned toward the Arab, who was looking at him blankly. Daru held out the package to him. "Take it," he said. "There are dates, bread, and sugar. You can hold out for two days. Here are a thousand francs too."
Humans are social creatures and do not want to be alone; nothing is more frightening than being separated from the pack. Except having to embrace free will. The Arab must decide whether to go to the safety of society, even if it is in prision; Daru must choose to live a lonely life, with no society and the constant threat of danger.