His death led to a violent succession struggle between his sons, which was won by Atahualpa, who in turn was captured and killed by Pizarro. And without that war, the Inca would have had a much better chance against the Spaniards, whose numbers were comparatively tiny. A similar thing happened to the Aztecs. The Moctezuma who eventually lost to Cortes was the nephew of a much more powerful king who died of smallpox. And the death of that great king encouraged some of the smaller states in the Aztec Empire to rebel, and some of them even fought for the Spaniards. And another side effect of disease was starvation, because there simply weren't enough people left to grow crops to feed the living. And then malnutrition made survivors that much more susceptible to disease.