Sometimes, aliens are big crazy monsters that want you dead. Other times, they're teddy bears that look like grizzlies. Racism is such a huge theme in fiction that it is constantly being reinterpreted and repackaged for us. What better way at capturing the idea of racism than by utilizing the ultimate outsiders? Aliens are foreign, look strange, and we usually can't understand them very well. Which is exactly why they're perfect scapegoats for humanity's hatred. Humanity just seems completely obsessed with hating on the aliens. And, in some cases, it's justified. In these cases...not so much...
10. The Day the Earth Stood Still
http://www.youtube.com/watch?v=OfpSXI8_UpY This is perhaps one of the most iconic and intentional uses of this trope. Alien Jesus comes down to Earth, and, upon arrival, is shot down by the trigger happy locals. All the alien wanted to do was just warn them about their nuclear weapons, but did they listen? Nope. Shoot first, ask questions later. That was the American way. Of course, the film was a commentary on the Cold War and McCarthyism without directly tackling the issues head-on. A political satire. Real people suffered because individuals' possible political beliefs were controversial. It became a witch hunt overtime, and cost many people their jobs and reputations thanks to McCarthy's kangaroo court. In the film, because the alien is foreign and otherworldly, it must be evil, despite there being no proof the alien is hostile. In fact, he's come to save us all from ourselves, and is willing to die -- and come back -- to salvage us from sin. We haven't changed much since the crucifixion...