In what way did Huayna Capac contribute to the decline of the Inca Empire?

The civil war was triggered by the death in 1527 of Sapa Inca Huayna Capac and his eldest son Ninan Cuyochi, who was the heir to the throne leaving no clear successor, both died of smallpox ... However, as Atahualpa was the son of the Inca emperor and one of his concubines he did not have legitimacy to the throne.