One way is to represent a person or being as representing perfect, flawless evil. Here's some examples in my opinion:
H. P. Lovecraft's Cthulhu. Lovecraft made the hair on the back of my neck stand on end when he described this being as having a degree of malevolence "not to be found in this galaxy".
Another example was the shark in the movie Jaws. The movie and it's background theme have long passed into humorous references, but when I first saw the movie, it's portrayal of the shark as encapsulating pure, relentless violence really creeped me out.
Also, another movie that really scared the daylights out of me when I first saw it was Alien - the alien creature was represented in the same way.