Why cruelty can run a whole world and decency can't
A destructive quality can be built into an institution, then into manners, then into the atmosphere of a place, while a decent one has to be performed by a named person every time.
Your antagonist's path costs them a specific human ability, and the difference between the two sides is not the size of the loss but whether anybody lets themselves look at it.
Your antagonist is paying a price too. They just can't see what it's costing them.
Your protagonist chooses their own cost and can name it, feel it and grieve it. Your antagonist's cost gets taken without your antagonist ever noticing.
And it's usually something they can no longer do, not something they own. They slowly lose the ability to connect, to know themselves, to be surprised by another person.
They read that emptiness as strength, or clarity, or being above weakness. Your reader watches the emptiness grow.
That difference does real work. It turns your antagonist from wrong into tragic. And it keeps your story's big question open, because both paths cost something real, and only your protagonist knows the price.
Colonel Fitts in American Beauty has given up any chance of ever saying one true thing about himself. The cruel part is that his rigid discipline is exactly what keeps him stuck there.
The difference between someone who's growing and someone who's stuck isn't how much they lose. It's whether they let themselves look at what it cost them.
So write two costs on purpose: one your protagonist chooses with open eyes, and one your antagonist pays and can't see. An antagonist who can name that cost and accept it anyway is closer to a second protagonist. That's a real story too, just a harder one.
A destructive quality can be built into an institution, then into manners, then into the atmosphere of a place, while a decent one has to be performed by a named person every time.
A sacrifice only lands when the story has already named what happens if nobody makes it.
Your protagonist and your antagonist begin inside the same destructive habit, and what separates them is that one keeps paying for the decent thing while the other found a way to stop.