Why did Pizarro kill Atahualpa's?

Francisco Pizarro killed Atahualpa primarily to consolidate Spanish control over the Inca Empire after capturing him during a surprise attack. Despite receiving a large ransom in gold and silver for Atahualpa's release, Pizarro decided to execute him to eliminate any potential threat to Spanish authority and to prevent a united resistance against the conquistadors. This act also served to instill fear among the Incas and solidify Pizarro's power in the newly conquered territory.