Evil is most difficult to resist when it is enveloped in beauty. No one is drawn to ugly. Beauty is in the eye of the beholder. Ugly too?

Some of the world's most successful confidence tricksters have been easy on the eye and show a pleasant, friendly manner. Evil can exist anywhere but it takes a practised eye to spot it in somebody who is attractive. 

There was a line in Lord of the Rings (the book, but perhaps not the movie) when Frodo decides to trust Strider. When asked why he replied, "If he came from the enemy I think he would look more fair and feel more foul." To which Strider (aka Aragorn) laughed and said, "So I look foul and feel fair?"