I recently read Lolita and disagree. He’s incredibly evil but I think you’d need to be a mass murderer to even be considered for the most evil. He actually backed out of drowning charlotte earlier in the book (although he was for sure involved in her later death) and I don’t think everyone would have that same hesitation. I’m a new reader and struggle to name worse characters but I’d say the wizard from wicked takes it. Or someone like Cthulhu if you want to take that approach. Humbert Humbert is scary because he’s a more grounded and realistic evil but there’s constraints that come with the realism