Creating the qualities of a protagonist and an antagonist are remarkably similar.
What defines your antag’s character? Does he thirst for power? Is revenge driving her actions?
Now give it a twist. What is the opposite of the attribute you’ve given him?
Assignment: Write a short paragraph that shows this quality in your antag.
Gollum looked at them. A strange expression passed over his lean hungry face. The gleam faded from his eyes and they went dim and grey, old and tired. A spasm of pain seemed to twist him and he turned away, peering back up towards the pass, shaking his head, as if engaged in some interior debate….For a fleeting moment, could one of the sleepers have seen him, they would have thought that they beheld an old weary hobbit, shrunken by the years that had carried him far beyond his time, beyond friends and kin, and the fields and streams of youth, an old starved pitiable thing.
The Lord of the Rings, The Two Towers - JRR Tolkien
Gollum isn’t the worst antagonist in LOTR but he is its main persona of evil. For just a moment, he questions his motives, feels regret.
Consider how you would show this in your antag. Define what your antag wants the most. Then image the opposite of that desire. Incorporate this in your MS.
Summary. Most bad guys aren’t evil all the time – with the possible exclusion of Sauron in LOTR. Give your evilness a twist and let your antag show a little humanity.