When I took my first screen writing class, the very first thing we discussed was a very popular misconception. A protagonist isn't necessarily the hero, and the antagonist isn't necessarily the villain. While in most stories, the protagonist is cast as the hero, and the antagonist cast as the villain, that doesn't make it universal.
I will admit, this post was inspired by a point Lindsay Ellis made in her recently review of "The Little Mermaid." In her review, she pointed out that King Triton is essentially the protagonist. He begins the movie as a pompous king, an overbearing father, and a bigot. By the time the movie is over, he's learned something. He learns to let his daughter go, and he's learned to stop being so bigoted. Throughout the course of the movie, he expresses regret at some of his actions. He gains enlightenment. While Ariel changes physically, she doesn't learn a thing throughout the course of this movie. She is essentially the same at the end as she is at the beginning. And Prince Eric was just there and bland.
I have made similar notes about that movie in discussion over the years. Lindsay is correct in her observations. Who is the protagonist of "The Little Mermaid?" King Triton. Who is the antagonist? Not Ursula. Ursula is the villain, yes. But Ariel inspires that change, she is the antagonist.
Now let's look at the first "Back to the Future" movie. I am going to make the same exact point here that my screenwriting teachers did, because they used this movie as their example. Who is the protagonist? Who grows? Who goes through the change and learns to be a better person? George McFly. He starts out as a spineless dweeb and learns to stand up for himself, and becomes a man. Who is the antagonist? Not Biff. As with Ursula, Biff is merely the villain. The antagonist of the movie is Marty McFly. Marty is the one who pushes, "antagonizes" George into going through this change. Marty is the hero of the movie, but he is not the protagonist.
And finally, let's look at "Star Wars." Throughout the first two movies, Luke Skywalker is clearly the protagonist, and Darth Vader is definitely the antagonist. This is not the case with "Return of the Jedi" which opens with Luke as a fully realized adult and Jedi. Who is the protagonist of "Return of the Jedi?" Who changes? You already know the answer. Emperor Palpatine is the villain of the movie, but who inspires the change? Luke. Luke is the antagonist and through his actions, he redeems his father and his father goes through the change. I'll admit, one of my central problems with "Jedi" is the fact that Luke isn't the protagonist. I don't mind the redemption of Darth Vader so much as I felt Luke's character development was sacrificed to get it. Look at Luke when "Empire Strikes Back" ends and look at him when "Return of the Jedi" opens. I think we missed a few critical steps in his character development.
Of course, George Lucas will tell you now that these movies are about Anakin Skywalker, not Luke Skywalker. But I never bought into the bullshit that is the "Star Wars" prequels, and I just outlined one of my many reasons why I never thought "Return of the Jedi" worked. But the rest is another blog post.
Once again, keep in mind what protagonists and antagonists actually are. They are not interchangeable with hero and villain.