When Christopher Columbus discovered America, he and his sailors were surprised to see the Native Indian practising old method and living in civilization. In the beginning, Spaniards were friendly to the native as they both exchanged goods.
Native American after coming to contact with the Spaniards became sick and died because of the diseases like smallpox, measles, chickenpox, plague, typhus, and malaria, which Native Indians did not have immune to fight.
With the arrival of conquistadors in New World, the relation between the Native Indian changed into hostile.
When Spaniards like Columbus, Cortes, and Pizzaro arrived in the "New World," at first their relations with native peoples were rather friendly. There were efforts to associate and negotiate with them. But the Spanish conquistadors ("conquerors") were always in a mindset of superiority and domination over the natives they encountered. If they sought the natives' trust, it was mainly so they could establish a relationship in which they, the Spanish, gained the upper hand. They looked upon the natives as a lesser sort of people and upon themselves as the bringers of proper civilization.
As Spanish efforts continued, they became more aggressive in their desire to gain wealth (gold or whatever other products they could exchange for such wealth). They enslaved natives and went to war with native tribes and nations.
Ultimately, the biggest way that the Europeans brought harm to the native populations, though, was in a way they didn't intend or understand. They brought with them germs and viruses from their continent that hadn't been known on the American continents, and so many natives became gravely ill and died. I've seen some estimates that say that smallpox killed up to 75% of the people in the Inca empire (in Peru) before the epidemics of that disease ran their course.
