First, "tone" refers to how an author/narrator feels about their subject matter or characters. So a question about tone would be stated: "How does the description of the region's landscape represent the tone of the author/narrator?

The way your question is worded suggests you actually mean "mood," which is the emotional feeling communicated to you by the story.

Tone and mood don't necessarily have to match. The tone may be detached yet the mood frightening. The tone may express fondness yet the mood express depression, etc.

If you need tone, I'd say "who didn't help matters either" indicates the author and/or narrator has a bitter feeling toward the subject of the story, thus has a bitter tone.

If you need mood, I'd say "the Region was, cruel to live in" indicates a feeling of desperation and despair being communicated to the reader, thus communicating a desperate and despairing mood.

You might want to think how the setting is explicitly related to the theme of absurdism, especially in the way that the natural landscape in the story is descrbed. Camus talks about the "benign indifference of the world" and this can be seen in the way that the setting is described and the way that man is set against an uncaring background in which mankind is truly insignificant.

The setting is intrinsic to the story of the Algerian colonist who finds himself caught between his obeisance to the colonizer and his law and his own loyalty to his people and the desert outside which he knows he cannot really live.  This "vast landscape he had loved so much" is a part of Daru; he becomes alone when the followers of the Arab prisoner write that he has betrayed the Arab because he no longer is perceived by the men as one of them.  By having associated with the foreigner, the Corsican gendarme named Balducci and signed for the Arab prisoner under the French Algerian government, Daru has lost his identification with his environment.