In "The Lagoon" by Joseph Conrad, what motivates Arsat to leave his brother behind?

Arsat's brother tries to drive back the Rajah's men, as they approach the camp with his gun. He is motivated to leave his brother behind when he sees that he has fallen and is surrounded by men. His desire to protect Diamelen prompted him to leave his brother behind, and he never forgot the way he shrieked his name ... three times.