What impression(s) did Europeans form of Native Americans, and how did that influence how they dealt with Native Americans?
How did Cortes and the European conquerors perceive the Native Americans?
"First Encounters: The Confrontation Between Cortes and Montezuma (1519-1521)," Chapter 1.
Since you refer specifically to the Spanish and the conquistadors, you have to remember that this was the world's dominant empire. Its citizens, for the most part, were devoutly Catholic, and believed that God had blessed them and their empire for their devotion with ruling the world.
As they first encountered Native Americans, who worshiped many gods, conducting rituals that appeared pagan or satanic to the Spaniards, and even conducted human sacrifices. So it is not surprising that Cortes and his men viewed the Aztecs and other natives as savages, or actually abominations before God. This helped them to justify their conquest of these peoples and their thievery of their lands and riches. They had the ultimate superiority complex.
Different groups had different impressions of the Native Americans. It depended a lot on which country in Europe they were from, what their religion was, and what their goals were in the new world. For example, the French tended to have a more sympathetic view of the natives because of the fact that a major goal of colonization was to profit from trade with the natives. This forced the French to have more contact with and develop a greater appreciation for them.
The Catholic church had also forbid turning the natives into slaves and took a great interest in them as "souls to convert."
English settlers seem to have viewed the natives largely as obstacles...they were more interested in cultivating the land that the natives lived on and came in greater numbers than the French, so it was more natural that they would come into conflict with them. Of course, certain Puritan groups developed local, hospitable relations with certain Native groups, but over time those natives had to be pushed out of the way.
The Spanish also were not allowed to practice slavery, but saw the natives as a resource to exploit. Men like Cortes saw the natives as tools to get what he wanted: gold. He would make allies with some groups, destroy others, and generally rampage through their lands in search of the precious metal. Though he was undoubtably impressed by the Aztecs capital city and their ability to oppress their neighbors, he found it hard to respect a culture that was so technologically (at least in terms of military hardware and metallurgy) inferior to his own.
I think you have to keep in mind, though, with a question like this, that it is very easy to get caught up in generalizations. There were quite a few people who appreciated the native's help, saw them as "noble savages," and were happy to live in harmony with them as much as possible. Unfortunately, these people seem to have been in the minority and the Native Americans were not given much respect by the European cultures as a whole.