He became emperor after defeating his younger half-brother HuÃ scar in a civil war that followed the death of their father , Inca Huayna Capac , from an infectious disease ( maybe malaria or smallpox ) . As an adult , Atahualpa became emperor upon defeating his older half-brother HuÃ scar in a civil war sparked by the death of their father , Inca Huayna Capac , from an infectious disease which may have been smallpox .