And Humbert Humbert is not only a terribly unreliable narrator, he's an incredibly unlikable human being.
Exactly! He is the villain... and one of the greatest in literature.
It's not like the book is trying to make you sympathize with and like this man who has a thing for little girls... quite the opposite.
Here I must disagree to a certain degree. Humbert is one of the most seductive villains is all of literature. The closest character I can think of is Milton's Satan, who like Humbert, can almost seduce the audience into liking him in spite of our better judgment. Nabokov is certainly playing with the reader here...