What does De Las Casas accomplish by including his comparison between the Romans' treatment of Spain to the Spaniards' treatment of Native Americans?

To show how poorly they treated them.

Explanation:

Bartolomé De Las Casas was a Spanish Dominican friar who tried to defend the rights of Natives after Spaniards conquered The Americas. He was the first resident bishop of Chiapas, in southern Mexico and also the first one to be named Protector of the Indians. To achieve his goal, he wrote A Short Account of the Destruction of the Indies, where he narrated all kind of terrible things done by the Spaniards conquerors towards the Native people. By comparing them to the Romans during the conquest of the Iberian Peninsula, De Las Casas showed how the pattern was repeated but now the Spaniards took the role of the conquerors and they were acting like they were treated before.