Atahualpa became emperor when he defeated and executed his older half-brother Huáscar in a civil war sparked by the death of their father , Inca Huayna Capac , from an infectious disease ( possibly smallpox ) .
He became emperor after defeating his younger half-brother Huáscar in a civil war that followed the death of their father , Inca Huayna Capac , from an infectious disease ( maybe malaria or smallpox ) .