I've seen a lot of people drag down mcu Thanos' character for his plan of a half genocide of the universe for being short sighted and/or incorrect in terms of what it would actually achieve, but that's not the point, the point is that he truly believed what he was doing would save the universe, and that's why he's such a fascinating villain.

(Word vomit alert)

Akemura Soga from kagurabachi is another pretty good recent example, though we have much less of him so far compared to Thanos, he's still a very interesting character. He is a man who weilds the Magatsumi. the most powerful of all 7 enchanted blades (they are a pretty big allegory for nukes in the series) and fought against invaders during a battle called the seitei war, where he and the other enchanted blade weilders, effortlessly destroyed the enemy forces, until the people of the island invading,got rid of the current leaders who were perpetuating the war, and managed to forge a peace agreement, until a couple years post war, akemura made the decision to completely wipe out the islands population by him self, resulting in over 200 000 deaths, years despite this, akemura stands completely resolute that he did the right thing, that's he protected his country from harm that they "were the enemy". Despite everyone calling him a monster and being locked up for what he did, despite the fact that he completely understands why his beliefs were viewed as dangerous. He still TRULY believes that what he did was the right thing, and that is so fascinating to me, though unfortunately we haven't gotten far enough into the series yet to truly know why he believes this, and what caused it.

Whether or not they are right isn't why they are a good villain, they are a compelling villain because they TRULY BELIEVE that they are right.