I made a lich NPC in one campaign who did what he did in order to obtain lichdom in order to end a war and save his country from invaders. He was basically "Dexter" evil. A sociopath. Like yeah, he's obviously a monster, but his motivations weren't the evil part, his actions were. He was reviled by his countrymen and hunted for the steps he took to achieve victory and peace and wound up sealing himself away deep in some forgotten catacombs feeling betrayed. The party actually met him while seeking for a rare object he'd supposedly had in his possession, and after centuries of reflection on the attrocities he'd committed and coming to terms with why his country had turned on him asked them to destroy his phylactery when they achieved their goals, which so happened to be said object.

He was a bad dude. There was no question. The shit he'd done stained his soul and even after all that he still didn't feel like the things he did weren't worth it, but he also hadn't done them out of malice or a sense of curiosity or real interest. He didn't care to return to the world or do evil. In a turn of cruel irony, his actions had brought about the rise of several death cults who wanted to copycat him purely to gain power, and this threw the country into a downward spiral before a holy order moved in and basically took over and built a whole new country.