Heroes and villains push each other. The protagonist's moral code, or lack of one, basically dictates the villain's response. A ruthless, ends-justify-the-means hero makes the antagonist feel justified, maybe even righteous. A paragon of virtue forces the villain into more extreme, theatrical schemes to prove their worldview. It's a twisted mirror.
I got thinking about 'The Locked Tomb' series. Gideon Nav isn't a saint, but her loyalty and bluntness create a foil for the necromancers' cold, calculating cruelty. Her existence annoys them, disrupts their plans through sheer stubbornness. The antagonist isn't just fighting her power, but her attitude, which is way more personal. That's what makes the conflict stick.
These dynamics aren't just plot; they're the emotional engine. You remember the fights because of what they reveal about both sides, not just the magic or swords.