Actually, one of the main cause that led to Atahualpa's execution was the false interpretations from Pizarro's interpreter Felipillo, which made the Spaniards paranoid. He told the Spanish that Atahualpa had ordered secret attacks & his warriors were hidden in the surrounding area. However, Hernando wanted to make sure first by going with a small army to check if the rumors about this hidden forces were true or not.
Unfortunately, a sudden trial for Atahualpa was held in Hernando's absence, eventhough there were men who were against Atahualpa's conviction, argued that the Sapa Inca should be judged by King Charles himself since he was the sovereign prince.
Among the charges were polygamy, incestuous marriage, and idolatry, all frowned upon in Catholicism but common in Inca culture and religion. At first it was decided that Atahualpa would be sentenced to execution by burning. He was really horrified, since the Inca believed that the soul would not be able to go on to the afterlife if the body were burned.
Friar Vincente, who had earlier offered his breviary to Atahualpa, intervened, telling Atahualpa that, if he agreed to convert to Catholicism, the friar could convince Pizarro to commute the sentence. Atahualpa then agreed to be baptized into the Catholic faith to avoid being burned at the stake, and later was given the name Francisco Atahualpa in honor of Francisco Pizarro.
(Colored woodcut: "Pizarro orders Atahualpa to be led instantly to execution" / 19th-century)