You know the fusty old white = good, black = bad dichotomy that influences everything from Westworld to the color scheme at funerals and weddings? Yeah, the White Witch doesn't mess around with that. She's all about the paler end of the color spectrum, but she's also 100% evil: the White Witch is a half-giant, half-genie who has taken control of the land of Narnia as a usurper.
Using her magic, she creates an endless winter in Narnia, but also prevents Christmas from coming. (Maybe she has something against eggnog?) She's a nasty tyrant who uses the Secret Police, headed by the wolf Fenris Ulf, and a network of spies to control everyone in Narnia. Her punishments are terrible, and she frequently turns her enemies into stone statues which decorate her house and garden. The Witch's greatest fear is that Aslan will return and crown four human beings, two male and two female, at Cair Paravel. When that happens, the legends say, she will be destroyed.
And guess what happens during the course of The Lion, The Witch, and the Wardrobe?
Part of what makes the Witch so creepily evil is that she's not what she seems. In the real world, we're used to appearances being deceptive. But in Narnia, the way things look is the way they really are, and the outside usually expresses the true nature of things. Aslan looks good and noble and wise because...he is. It's not hard to guess that creatures like centaurs and dryads are always good. And it's pretty darn obvious that other creatures, like hags and wolves, are bad.
The Witch, though, is a deceiver. She appears to be human, but she definitely isn't. Mr. Beaver explains to Peter that the Witch bases her claim to the throne of Narnia on the assertion that she is human.
"But she's no Daughter of Eve. She comes of your father Adam's […] first wife, her they called Lilith. And she was one of the Jinn. That's what she comes from on one side. And on the other she comes of the giants. No, no, there isn't a drop of real Human blood in the Witch." (8.35)
The Witch's ability to simulate humanity without really being human is part of her uncanny evil. In the same way, the whiteness of her face is eerie. Instead of simply having pale skin,
Her face was white – not merely pale, but white like snow or paper or icing sugar, except for her very red mouth. It was a beautiful face in other respects, but proud and cold and stern. (3.32)
Like the world that she creates, the witch is pale and cold, practically lifeless, and lacking in both passion and compassion. In fact, the only passion she ever seems to feel is anger.
Although she's evil, the Witch also seems to be a necessary part of the structure of morality in Narnia. The Deep Magic from the Dawn of Time grants her certain rights. She reminds Aslan:
"You know that every traitor belongs to me as my lawful prey and that for every treachery I have a right to a kill." (13.41)
This suggests that the Witch has been in Narnia from the very beginning and that she fulfills an important function as the person who punishes betrayal. We learn more about the Witch's origin and history in another of the Chronicles of Narnia, The Magician's Nephew. (Her name, if you wondered, is Jadis.)
What we discover in The Lion, the Witch, and the Wardrobe is that, although the Witch is evil, she can't force other people to do her bidding if they resist her. Sure, she can punish or hurt them, but they still have choices to make. When the Witch feeds Edmund her enchanted Turkish Delight and convinces him to betray his brother and sisters to her, she's only playing on his preexisting selfishness and greed.
So maybe the most frightening thing about the Witch is that she draws out the evil that exists in people to begin with.