Daru is disgusted by the Arab's act of violence, so we might conclude that Daru is dedicated to peace. This is a possible reason he refuses to get the gun in the middle of the night. Daru is French but had been born in Algeria. His official allegiance is with the French but his refusal to turn the Arab in indicates that he will not take a side in this situation. He feels part of both or torn between the two. And perhaps, in general, Daru doesn't want to take a side in the "us vs. them" of the French-Algerian struggle.
Given Camus' philosophy of Absurdism, we might conclude that Daru does not take a side in this sense because to do so is absurd or meaningless. Doing so simply means he continues to take part in an ongoing cycle of violence. Daru tries his best to do the compassionate thing, but he feels stuck in a meaningless situation:
Towns sprang up, flourished, then disappeared; men came by, loved one another or fought bitterly, then died. No one in this desert, neither he nor his...
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