A while back I was reflecting on Star Wars, and in particular the transformation of Anakin Skywalker into Darth Vader. It occurred to me that Vader is the "bad guy", so Anakin becomes Vader despite the audience's wishes—but wouldn't it be great if the audience watched a descent into evil and actually rooted for it the whole time? That would be an achievement indeed. Then I realized that this movie has already been made: it's called The Godfather, with Michael Corleone in the Anakin role. In Part II, we watch Vito Corleone go through the same transformation, and watch Michael descend even further. And, amazingly, we never stop liking them.
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