Evil skin
Since outward appearance is supposed to reflect the worth of the soul, American movies (drawing, of course, on a long tradition of European folktales, literature, and drama) often portray evil characters as being ugly or deformed, or having features associated with particular racial groups (such as hooked noses). I just stumbled across the Manifestation of Evil page, which shows lots of examples of skin conditions, scarring, unusual medical conditions, and tattoos being used in movies to represent evil.
My favorite line:
Off the silver screen, accessory nipples, like other birthmarks, do not indicate evil.
Of course, there's an opposing tradition in which a fair outward form conceals evil within (isn't there a Shakespeare line about that? I'm not finding it), and in some cases even ugly people turn out to be good guys. But I do find it interesting that certain kinds of facial features and medical conditions are widely used as shorthand to let audiences know that a character is Evil.
(The search that led to that page was sparked by learning why the character played by Tiny Ron in Rocketeer looked so familiar: he was made up to look like classic acromegalic horror-movie actor Rondo Hatton.)
