Commonly in science fictional writings, aliens are thought to have two different races of "good" and "evil" aliens if not more races of other species. This concept is often depicted in science fictional novels, theatrical films, television programs, etc.
Some believe that aliens have different affiliations and alliances with other governments or other races from different planets, if there is any. The concept of aliens choosing different alliances most likely comes from various forms of media, especially theatrical films and television programs.
Some believe there are "good" aliens that have come to the planet Earth to protect the human race from the "evil" aliens who have come to "take over the world" and destroy the human race.