Daru's behavior with regard to firearms suggests that they have no place in his life. The narrator states that Daru has been to war, when he describes that the quiet on the plain after the war was hard for him to get used to. So with experience of war, with the death and suffering, it is easy to see why Daru, a teacher, one who uses words and not a sword, would be repulsed by weapons. He expresses his anger toward men and their need to fight and kill. Daru will fight to survive, but violence, even the aggressive action of taking the Arab to jail, upsets him.
By making the Arab a murderer, Camus provides a valid reason, an extreme, for Daru to protect himself, but here we see again that Daru is not a man of violence. He is happy with his lot in life, even in the midst of such hardship on the plain, in the middle of a drought. He thinks not of an impending revolt, as Balducci suggests. He thinks of his students and the harshness of their existence in this land he loves so much.
Logic tells us that if the Arab has killed someone, Daru might be in danger. But Daru's refusal to carry the gun tells the reader how uncomfortable the weapons make him feel, how committed he is to a peaceful existence, and the strength of his conviction in his refusal to be swept up into a world where guns have a place.