how does shooting 4 additional bullets into the Arab jive with him living and operating from a life of indifference? He is making an affirmative action. Yes, he may have taken one path or another, but once there....

Indeed this is a problem with many possible interpretations, but I'd recommend invoking Camus' own idea of "rebellion" here. The sun is the famous "absurd reason" for murdering the Arab, but remember that Camus does not advocate a quite resignation to the absurd. On the contrary, he urges us to fight and rebel against it--carving out a meaning for ourselves. I think it's possible that this act of shooting the Arab four additional times might represent a frustrated attempt to do just that.