Why does Mersault kill the Arab?

After reading the pages assigned, I think that Mersault killed the Arab because it was a natural reaction for him. He felt threatened by the Arab and though that he was going to be harmed, really in the end he was indifferent to the idea of killing and merely did so because he thought he was going to be harmed. I think it's interesting to note that Mersault only does certain things out of self interest and social obligation, he thought that the Arab was going to hurt him and the others so he shot the Arab. As a result of this scene, I believe that Mersault can't tell the difference between right or wrong, his sense of morality is skewed. I think the leading theme behind this scene is indifference, he killed the Arab simply because that's what he thought would solve the situation.