Why do people in the U.S hate Mexican people?

Mexico in the Pre - Columbian era were not Hispanic. They were Native Americans. When Cortez, Columbus and the rest of the explorers "explored" in other words killed off thousands of native Americans and even fed the babies to dogs. Why do people in the U.S hate Mexican people. Why are there borders when California, Texas and Arizona are parts of Mexico? Why do think of them as dirty or people who steal jobs. Why do they think of Mexicans as "wetbacks" or "beaners" I find it disrespectful when the same people who say "This is the land of opportunity" or "The land of the free home of the brave" disrespect the people who are indigenous to this continent.