I've read a decent amount of fantasy where there's wizards on the sides of both good and evil casting magic.... and you know what? A lot of times, the good guys use the same spells as the bad guys. I've always thought that magic, if such a thing actually existed, would be a tool, nothing more. It's like a gun....a gun is not good or evil in itself, it can be used for either.
Or a good example could be in a favorite series of mine, Xanth, where people pretty much all have unique magic that only they can use, but the only thing that makes it good or evil is how they use it. And for that matter, there a lot of examples in that whole series where "good/evil" is matter of perspective.