It's an intriguing storytelling device that can add depth to a seemingly familiar tale. I've previously written about using existing villains as protagonists in new stories, but I'm talking about retelling those original tales from the point of view of the villain.
In the old days, audiences didn't care about the bad guy's motivation, it was just good versus evil. At the same time, I don't enjoy seeing writers go the other extreme when they make morality too relativistic, whitewashing the horrendous deeds committed by the former antagonists.
Still, Wicked (both the book series by Gregory Maguire and especially the Broadway musical adaptation) did an excellent job of fleshing out the Wicked Witch of the West from the Wizard of Oz story.
Other authors have explored the concept of revisiting classic tales from the villain's P.O,V., whether it's Moriarty from Sherlock Holmes, Grendel from Beowulf, or the Big Bad Wolf from either Little Red Riding Hood or the Three Little Pigs.
My biggest disappointment with the Star Wars prequels was that nothing fundamentally changed with what we saw of Darth Vader in the original series, besides seeing the boyish face behind the mask. It would have been interesting to see more of the Emperor's motivation and what attracted him to the Dark Side of the Force, besides the stereotypical lust for power. I guess I was expecting some sort of twist like in The Empire Strikes Back. A "retcon" (retroactive continuity) of the saga with new understanding of the villains' actions would have made us see all the Episodes in a different light.
Are there any other tales that might benefit from a reimagined examination from the antagonist's side?