For me the question is ultimately this: Could I see this person existing in real life?
To give an example, the reason I can buy into Hellsing’s Major is that despite the fact he is clearly evil. There is a spark of charisma, you can see why people would be drawn to the idea of just doing what they want (wage war) without caring about the consequences. In contrast, I feel the reason pure evil started falling out of fashion is because it became cartoonish. People could not for the life of them understand why anyone would ever follow stock villain #36 nor could they understand how this guy ever reached a position of power in the first place. If someone is evil, a dick to his employees and makes foolish mistakes it’s hard for me to engage with the story.
If you watch Young Justice and see The Light (villain league), Lex Luther’s portrayal is picture perfect. He’s clearly evil and self interested but he’s so competent and charismatic that you can see why people would still work for him.