what does arsats brother do while arsat and diamelen run to the canoe, and 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.