Genocide happens due to a variety of reasons, including extreme prejudice against other cultures or races, propaganda, fear of other cultures or races, a heightened sense of nationalism, and ill-informed ideas on how to conduct social change. Additionally, some cultures try to create legends of their own ruthlessness.
During the years leading up to World War II, the Nazi party in Germany used propaganda within the newspapers and radio to vilify the Jewish people and restrict Jewish influence in the legal and medical professions. These measures helped put a spectre of guilt within the public mind about their Jewish neighbors.
In genocide cases in Cambodia and Rwanda, the culture being eliminated was described by those in power as vermin, cockroaches and sub-human. The destruction of these cultures was then spun as being good for the betterment of society.