Make him three dimensional and give him, as <PERSON> says, a backstory to explain his journey into the shadows.
The character might just see the big picture and think the hero is naive and no different from how he was once. He will not notice the corruption of his soul as it was gradual and seemed necessary to attain his goal.
You could give him moments of regret and he might misunderstand his own situation just enough that he believes he might help the hero by teaching him what experience taught him.
Villians who are one note are boring. Those that are complex and have thoughts beyond destruction are more chilling and intriguing as they are more realistic.
Sometimes the villain is right. <PERSON> in Watchmen did what he did to promote world peace. <PERSON> in Batman vs Superman is presented as a man who saw the big picture and might actually be right about potential risk of having an alien living among us.
Give your dark one some thoughts outside of plotting, something beyond ruling with an iron fist. He might have learned the necessity of such from history. The Tsars, for example, tended to alternate between a ruler who wanted to improve the lot of his people and failed - often assassinated - to tyrants who saw their kinder father killed by the people he wanted to help. His son would rue the lengths to which his father went and be lenient, repeating the cycle.
Make him a person who has strayed off the path and not noticed that the darkness he sees is within him. 627
Give him something to hold onto, something beyond the usual. Give him some humour and maybe some charm. His actions are terrible, his choices impossible, but he himself believes that such must be done, sacrifices made for the greater good.
In one novel I was working on, I had my hero hating my villain. He hated everything he stood for and he would strive to foil him at all points, knowing himself to be the hero. He was the mage wielding white magic and this black sorcerer was evil incarnate.
Only, I discovered he was wrong. My villain was serving the greater good, making sacrifices that seemed impossible but were necessitated by the actions of the hero, who was wreaking havoc with the best of intentions. I had a villain who was serving the good and a hero who was paving his road to destruction with the best of intentions. I liked this better, the supposed hero being an inadvertent villain and the feared villain holding everything together with everything at his disposal. 425