Humbert the Pedophile is quite clearly a detestable rapist, but why did Nabokov choose him as the vehicle for this story? When you get past the discomfort of the actual story itself, Lolita paints a portrait of a man suffering from the dilemma of his morals versus the morals of society around him.
The underlying idea is moral relativism. Morality is subjective to the cultural, social, or even personal background you are from. We are quick to dismiss Humbert’s desires as taboo and wrong, but who are we to judge? There is no universal moral standard that we all naturally follow from birth. Our unique environments impart to us moral values, and no set of values is more right than another. When we start banning books and censoring based on personal moral standards of right and wrong, wholesome and obscene, we are imposing our morality on others. Nabokov is in no way supporting pedophilia. Lolita is an extreme example used to illustrate the point that right and wrong are in the eye of the beholder.