Part One of Two: Protagonists and antagonists, heroes and villains.
There are two sets of terms for writers to remember in this context:
one) protagnists and antagonists.
two) heroes and villains.
Almost every work of fiction involves at least one protagonist who the audience is intended by the writer to more or less identify with, who struggles against various obstacles which are called antagonists. Antagonists may be inanimate conditions, or animals, or other people.
It is perfectly possible to write a story about a very minor conflict between people. For example, suppose that a child tries to get a treat that is unauthorized by their parent, and their parent tries to prevent them. That is a real type of minor conflict which happens many thousands or even millions of time every day.
And it seems to me that a writer could equally well make either the child or the parent the protagonist, which would make the other one the antagonist.
But considering the relatively minor stakes in such a conflict, it would be very hard for the writer to depict the protagonist, either the child or the parent, as a noble hero, or the antagonist as an evil villain.
Heroes are fictional characters who fight for good causes against evil causes in high stakes conflicts where a lot is riding on the outcome. Villains are fictional characters who fight for evil causes against good causes in high stakes conflicts where a lot is riding on the outcome.
And the thought has occurred to me to wonder who determines which cause in a conflict is the good cause and which is the evil cause.
Is it the writer of the story, or the reader of the story? Or maybe some hypothetical god who mighthypotheticaly exist and hypothetically punish writers and/or readers of stories where the protagonists are evil according to the god and the antagonists are good according to the god?
So the protagonist of a story might:
One: fight against natural forces which are antagonists and not villains.
Two: fight against other persons, antagonists, in a situation where there are no heroes or villains.
Three: be a protagonist and a hero, fighting against antagonists who are evil villains.
Four: Be a protagonist and a hero, fighting against antagonist who are as good and heroic as him but on the other side of a conflict.
Five: Be a protagonist and a villain, fighting against antagonists who are also evil villains.
Six: be a protagonist and a villain, fighting against antagonists who are good heroes.
And if a writer is a believer in an evil cause, he might intend to create a protagonist of type three, but succeed in creating a protagonist of type five or type six.
A number of historical characters are depicted in many different stories, movies, and tv shows. And it is easy to find examples of a character who is depicted as a heroic protagonist, which their antagonists evil villians, in some movies, episodes, and stories, and as an evil villain fighting antagonists who are noble heroes, in others.
Part Two: An example from the Silmarillion.
A good example of a plot partially similar to the plot in the original question, is the story of Beren and Luthien in the Silmarillion of J.R.R. Tolkien. It is sort of a high fantasy version of the struggle of Tolkien and his wife to overcome the opposition of their families to their relationship.
Anyway, Beren, a mortal man, and Luthien, an immortal elf princess, are the protagonists, who struggle to overcome all obsticals to their love. They face many antagonists who seek to prevent them getting together, including Sauron, the dark lord in The Lord of the Rings, and his master Morgoth, the original dark lord, and Carcharoth, the terrible wolf, and elf princes Celegorm and Curufin, sons of Feanor. All of the above are more or less villains in the story of Beren and Luthien, though Celegorm and Curufin, and other sones of Feanor, are sometimes protagonists, sometimes antagonists, sometimes heroes, and sometimes villains, in other stories in the Silmarillion.
But the main antagonist Beren and Luthien face in their story is Luthien's father, King Thingol of Doriath. After Luthien makes Thingol vow not to harm Beren, see brings him before Thingol, and Beren asks his permission to marry her.
And Thingol reacted like many an over protective father in real life and in stories:
But Thingol looked in silence upon Luthien; and he thought in his heart: 'Unhappy Men, children of little lords and brief kings, shall such as these lay hands upon you, and yet live?' Even though Thingol's own wife, Melian, is at least as high above Thingol as Luthien is above Beren, Thingols fails to recognize the precedent or see any justification for Beren's request.
So Thingol says he will let Beren and Luthien marry, if Beren brings him a silmarill (one of the three mystic gems bound up with the fate of the world that the Silmarillion is about). All Beren has to do is get a silmarill from the Iron Crown of Morgoth, the most impossible task anyone was ever given in the history of Middle-earth. (There is a scene in The Lord of the Rings where Frodo and Sam mention how much more dangerous Beren's quest was than theirs). And everyone sees that Thingol wants to send to send Beren to inevitable death without breaking his oath not to harm Beren.
And when Melian warns Thingol, he replies:
I sell not to Elves or Men those whom I love and cherish above all treasure. And if there were hope or fear that Beren should come ever back alive to Mengroth, he should not have looked again upon the light of heaven, though I had sworn it.
Thingol's motivation is perfectly understandable to any father who has ever thought that the boyfriend or suitor of his daughter wasn't good enough for her. But sending someone to certain death merely because you don't approve of members of their species courting one's daughter is a rather villainous act.
Imagine how Star Trek fans would feel about Skonn, father of Sarek, if Skonn had forbidden the Human woman Amanda Grayson to marry Sarek unless she completed some feat as impossible and deadly as Thingol gave to Beren.
Thingol appears in other stories in the Silmarillion, and more oftain as one of the protagonists and/or heroes instead of an antagonist and/or villain.
And the main characters of the Silmarillion are treated a lot like historical characters, sometimes being protagonists, sometimes antagonists, sometimes heroes, and sometimes villains, in different stories.