Darth Vader was an epic villain precisely because 1) his prowess in combat was far superior to the protagonist’s, and 2) his internal conflict was hidden until the very end of the trilogy.
Um… who cares? Seriously, why must a villain only be conflicted at the end of his life. Look at Macbeth, the entire story is about his internal conflict.
Macbeth was not a villain. He was an antihero.
A morally conflicted antihero can make a compelling protagonist.