Sometimes the Evil is comically used, to indicate what team the characters are on. The thing is, it gets into tension with the terms good and evil.
For instance, in the book, Molly does some things such as lie and pose as someone else, which are regarded as wrong -- even though she is, of course, the Wicked Witch. She also doesn't think that she can use a book of spells from a White Witch. One would think that bending White Witchery to her wicked purposes would be an especially evil deed. It's not just a matter of evil having some standards; the wicked things she can do are at least as evil.
To be sure, one would need limits and rules for the Wicked Witches and Evil Sorceresses and Mad Scientists to work, more or less, together. The book makes a point that they don't rule the world because they do not trust each other, but they manage to harmonize well enough to survive. But insofar as they do so, they are not Evil. (Like in The Invisible Hook's discussion of how pirates had to work peacefully and harmoniously together.
Then, of course, under the influence of D&D and such like, things get interesting. I have heard descriptions of Evil saying that just because you're Evil doesn't mean you can't have friends, which is wrong. Insofar as you have real friends (xkcd explains the distinction here) you are not evil. This is one of the flaws in the alignment system consisting of taking every moral dilemna that the wisest and most intelligent souls have broken their hearts over for millennia, misunderstanding half of it, boiling it down to a rule system, and then handing it over to sophomoric players (many of whom do have the excuse of being sophomores).