The demonization of extraterrestrials is a complex matter since many people had negative experiences with several alien races. Because of these experiences, researchers accept the view of aliens as evil entities.
As a recommendation for you, I recommend you to read Sam Keen’s Faces of the Enemy, which offers a lot of detail about this topic. (see video below) It seems that mass media wants to impose a negative or demonic representation of aliens on us by means of several stereotypes. You only have to look at Hollywood movies such as Independence Day, War of the Worlds, etc. We are also flooded by best-selling books featuring disturbing abductions experiences.
What we need to do is avoid generalization when dealing with this kind of experience.
We need to take into account that those devil aliens are only a small part of a much larger group of alien races. It is clear that what media wants to is to generalize the stereotype of bad aliens as the next major enemies of our civilizations.
Have a look at the following video for more information and please don’t forget to share your opinions with us. The following video summarizes the main ideas of Keen’s book.