Give this lady a bike and a theme song and she might as well take over for the Wicked Witch of the West – except, instead of your little dog, she'll get your little mouse, too.
When we first meet the Grand High Witch, she is young, pretty, and stylin'. That doesn't last long, though. When she takes her mask off, she becomes the most atrocious thing on the planet. The narrator uses a whole bunch of adjectives and similes to describe her, but he pretty much sums it up with "worm-eaten" (7.7). Nothing alive should look like it's worm-eaten.
The most important thing to remember about the Grand High Witch is that she's just plain mean. She sure doesn't try to hide it, either. She's always yelling, calling her fellow witches "idiots" (9.36) and "blithering bumpkin[s]" (8.34) and other things of the sort. When you think about it, though, any creature who can kill a child is clearly mean – and then some – so why is the Grand High Witch any different than the other witches? Well, in some ways, she's not. She's just our scapegoat – we blame everything on her.
Here's the thing: witches have the capacity to hate an entire group of people (children) but most humans don't. Humans – at least the kind of humans who'd take the time to read a classic children's book – tend to only have such strong feelings toward one person at a time. Someone from Nebraska might make us really mad, but we don't assume that everyone from Nebraska is a jerk. That's all to say that Roald Dahl might be giving us the Grand High Witch just so we have someone to focus our hatred on. So that there's a clear-cut bad guy, or, should we say, bad gal.