The correct answer to this open question is the following.
Members of groups such as the Ku Klux Klan believed that the whites were a superior race than African Americans. That is why the violently attacked black people.
The Ku Klux Klan was a white supremacist group that did not like black people for considered them an inferior race that did not belong to the United States. The hate this group expressed derived in aggression not only to black people but other minorities that emigrated to the United States. Historians consider that there have been three general phases in which the Klan has operated. From its foundation in 1865 to 1871, from 1915 to 1944, and from 1947 to the present day, when they basically express themselves to political organizations.