The Black Legend is a claim against the Spaniards by the English, because they were seen as excessively bloodthirsty, cruel, greedy,and heartless. For all of their bravado, they were also considered to be an inferior, impure race, because they were a combination of African and European. They also had Muslim and Christian roots. The Spanish Empire was considered to be the empire of darkness and barbarianism.
The Spanish came to America to gain riches and to increase their social standing. When the Spanish came here they were already a very racist people. Also, they didn't plan to stay forever, so what reason would they have for trying to make friends. They came here expecting to find what Cortez found, large cities made of gold, but except for the Incas and Aztecs that was not the case. When they discovered that there was no gold to be found, they were very upset and they took it out on the Indians that already lived here. .
When they came upon Indians that they considered to be worshiping differently than they were, they were obligated and encouraged by the church to "set them straight" using any means necessary. They seemed to overdo it. For example, when Onate was colonizing the Pueblo country, he sent some soldiers ahead to tell the Pueblo tribe that they should now consider themselves subjects of the Spanish crown. The Indians decided that they were not going to be Spanish subjects and killed 13 soldiers. Onate decided that he would punish these Indians. He killed most of the Pueblo warriors and took captive at least 500 women, children, and men that were too old and sick to fight. Of the Indians he took captive, about 80 that were over 25 were forced to serve as slaves for a period of 20 years and have one foot amputated. Personally, don't see how a one-footed slave is very useful, but I guess they can't escape that way. The women and children were also forced to be slaves, but were not mutilated like the men were.