The author Bartolome de las Calas was a Bishop of Chiapas and a Catholic priest. The thesis of his work Destruction of the Indies is that Spain might come under divine retribution for its treatment of the Native Americans in what was then known as the West Indies. During the time period in which de las Calas was active in the West Indies the Indians were being used as cheap slave labor and their conditions were utterly appalling.
His purpose was to expose these atrocities and to condemn the hypocrisy of Spanish conquistadors who were supposed to be good Christians yet were brutalizing the natives as if they were not human. He was also trying to counter the writings of other authors who were saying that the natives were uncivilized savages and the Spanish were within their rights to treat them so poorly.
Destruction of the Indies was written by a Catholic priest. Thus he dwells upon the responsibility of the Catholic Spaniards to be good Christians to their fellow man. He believed that God in heaven could see the plight of the natives and would soon deliver his divine retribution upon the Spaniards for their cruelty.
He was also trying to debunk the idea that the natives were savages and uncivilized. De Las Calas was perhaps one of the first writers to propose the idea of the 'savage noble' that even if the natives did not have Western customs they were still people and did not deserve to be treated like animals. At minimum the article espoused the idea that the natives had rights too and that those rights were being violated by their treatment at the hands of the Spaniards.
Finally, his work may have been somewhat exaggerated or at least slanted. He was trying to expose the ill-treatment of the natives and it is possible that he may have exaggerated his accounts of the maltreatment to make it appear more shocking and worthy of attention. He may have had political motives in making this expose so he might have slanted the expose a little.