Answer:
The Spanish empire wanted to enslave the Native Americans and treat them harshly, in order to extract from them as much free labor as possible, and as much economic revenue as possible.
Spanish clerics did not agree with this view, specially Bartolome de las Casas. He believed that the natives were people worthy of rights, as everyone else. He thought that they should be converted to Christianity, baptized, and respected.
De las Casas view prevailed, and the Spanish Empire formally prohibited the enslavement of Native Americans. However, many labor schemes that were devised like the Encomienda, made Native Americans almost slaves in all but name.