Las Casas brought attention about the massacre of the Amerindians to help bring doubt to the character of the Spaniards. Common actions of the Spaniards as described by Las Casas "divided a man into two" (17), "snatched young babies from the mother's breasts, and then dashed out the brains of those innocents against the rocks" (17) to show the acts that can only be committed by uncivilized men.
Las Casas, wanted the king to become untrusting of the Spaniards, also mentions the possibility that all the gold was not making it back to the king "naturally addicted to rob, thieve and steal; thus under pretext of serving the king, they dishonor God, and defraud his imperial majesty" (104). I feel this is the true agenda of Las Casas and the one of the reasons he brings up the massacre to the king.
The other reason is he cannot keep these horrendous acts to himself. As a man of God, Las Casas is bound to speak against actions that offends his god. His moral objections were against the destruction of potential Christian societies and the slaughter of villages that did accept Christianity as their one true religion. Las Casas mentions the conversion "reducing them to the knowledge of Christian faith they were received by the inhabitants of this kingdom, that survived the heat and rage of the Spanish cruelties" (62-63)
Las Casa did not view the Amerindians as fully human. He does acknowledge them as people, but in his mind people without the acceptance of his God are just not equal to them. This is shown throughout the book in passages according to Las Casas "go and teach all nations that infidels, living peaceably and quite in their hereditary native country, should be imposed upon pain of confiscation of all their chattels, lands, liberty, wives, children, and death itself, without any precedent instruction to confess and acknowledgement of the true god" (32).
This book was extremely detailed in the actions that took place when the Spaniards arrived in Hipiola. It is such brutal open honesty of the horror that occurred. However, I feel that the author even though he tried to appear as a heartbroken man due to these injustices, did not truly care that these people were brutally slaughtered. He would have only truly cared if they were converted Christians that followed his own beliefs.