Why is the line, "I'll never forgive you!" so prevalent?
It doesn't matter who the villain is or what they've done. I always see it.
It feels really out of place if say, some boss is bent on total destruction of the world and your characters let them know that there is no chance for forgiveness. You would think that's a foregone conclusion at that point. Were they ever going to ask for it in the first place? I really doubt it.
It's a silly line, but works because of course they won't. If they did, it sure would paint the character who said that in a bad light.
Imagine the Big Bad somehow had a change of heart, asked for forgiveness, then got decapitated by the hero.