He came to power by overthrowing an even more tyrannical figure, and his conquests are in an attempt to make his people comfortable. His kingdom, despite all of the evil iconography and dark lifestyle choices, is prosperous, and his people love him.
His frustration turned into a burning hatred of diverse groups, who on one hand preach diversity while cursing those who are diverse simply by being different. They were a human fighter long ago, and they were cast out from every group they tried to join for being so one note and useless. Perhaps the only way to defeat the BBEG is to kill this hero, or have the hero accept that part of themselves. It could be revealed that in order to become so good and virtuous they viscerally expelled all evil from them in a painful magical manner, and that evil eventually gained consciousness. They are actually the dark side of this great, fabled hero who was so great and virtuous that tales about him are still told to this day. They think they are fighting an even worse foe, and they believe the party are agents of this foe. They never associate with beings known for being evil by nature. They have a deeply held sense of honor, never attacking the party when they are clearly outmatched even to the detriment of their own plans. They only really target one person, and when the party finds out who and why, a trusted ally is revealed as the person who ruined the BBEG's life. They're actually a really good boss, and their minions have no real complaints about working for them. Mentioning a certain name in their presence will cause them to become visibly sullen and withdrawn, as soon as their alone, they burst into tears (Bonus points if they only really show one or no emotion throughout the campaign)