Camus makes the Arab a murderer to influence the reader's response to the dilemma Daru faces in the story, based upon Camus' political concerns at that time between Algiers and France.
The reader expects Daru to find the Arab repugnant so that when Daru is "ordered" to take him to jail, we expect that Daru would follow directions without a second thought. First, because the Arab has committed a terrible crime (murder), rather than just assault or thievery. Second, Camus makes the prisoner an Arab because at the time he wrote the story, the Arab's predicament would not have elicited much sympathy, as Arabs were not generally accepted or respected, so if Daru was mean to him, few would have cared, but might even expect it. Camus makes a strong personal statement in treating the Arab like a "guest" in light of this.
Centrally, Daru's refusal to take up any kind of gun against the Arab shows Daru's refusal to take sides in the conflict between the French and Arabs. This is Camus speaking to his personal refusal to choose sides in the problems between the Algerians and the French.
Ironically, Daru cares more for the Arab than the Arab's own people: Daru cares for him in his home, and offers him the chance to escape, even though the Arab does not take the opportunity. All the Arab's friends do is threaten Daru, blaming him, instead, for the Arab's incarceration.