Wicked: or using other's characters

So, I was just wondering,
Do you think it's okay to take someone's characters in your own story (not so much a fanfic here) and turn them 180 degrees?

I started thinking about this with "Wicked," the Broadway musical/book. I think the music's okay, but I hate the story, mostly because I'd hate to see my characters treated like that, completely turned around. My bad guys are bad, unless I make them misunderstood!
What do you think? Is it right to change someone's characters?

p.s. I don't mean traditional characters, like those from fairy tales etc. Original characters from publicized works.

I find being able to turn characters around 180 degrees is the sign of characterization done perfectly.

Bad guys don't think they themselves are bad. They see themselves as the good guys. They have motives and ideals just like the "real" good guys we focus on in the story.

My characters have all had a short sketch from their perspective done, if only in my head. I know what they want and why they're doing what they're doing. That way, they're people. They're not supervillians who want to take over the world just cause they can. They need honest sadism and a thirst for power to be able to do that. Then you write them from the other perspective and the story makes the hero look like a villain.

I think Wicked shows that there was room in the characterization to switch it around. No character was outright bad in the story, so it made it more realistic. I noticed during the production that every event from the original story had an explanation, and stayed the same. It made me enjoy The Wizard of Oz more, because it just looks more real and less black and white cheesy.

No, I understand the whole perspective things- everybody has a drive of some sort keeping them going. But what I meant was if you had a character that you made bad (even though from their point of view they're not) or good or whatever, would you want someone else changing it around?

I think, in Wicked, they had a certain point, a certain moral they wanted to get across, and there's nothing wrong with that. But I guess it just bugged me that they were using someone else's established characters to do that, instead of just making up their own characters.