You can always spot the ones who've read 'The Lord of the Rings' properly versus the ones who just watched the movies when this trope comes up. Sauron's whole thing in the books is less 'evil guy wants power' and more like a melody that's gone permanently out of tune—he can't comprehend harmony anymore. That's the inner conflict stripped to the bone: a being so far down a path that the concept of turning back is literally incomprehensible.
It's not about choosing good over evil in a moment; it's about whether the architecture of your soul even has a door left to walk through. Redemption arcs for dark lords often fail because writers don't grasp that scale. Making them 'sorry' feels cheap. The interesting attempts, like some fan interpretations of Voldemort's final moments or certain grimdark fantasy anti-heroes, hinge on a sliver of self-awareness breaking through millennia of calcified malice—not to save themselves, but to maybe stop the mechanism they've become. That tiny, futile crack is where the theme lives.
Honestly, I think the trope works better in romance-adjacent stuff now, where the 'dark lord' is a love interest. The conflict is externalized through the protagonist's belief they can be saved, which is its own fascinating mess.