It's easy to forget that the entire memoir is supposed to present a realistic and historical account of the relationship between Humbert and Lolita and how it led to the murder of Clare Quilty. Throughout Lolita, Humbert, our criminal narrator, refers to the readers as his jury, making appeals and presenting "evidence" to gain sympathy. He does make gestures toward admitting that he is a monster and a maniac who deprived Lolita of her youth, but he also dares the reader (and the jury) to prove it. In other words, he never takes full blame. From the beginning we know he is imprisoned for committing a crime, but the exact nature of it remains unknown. Did he murder Charlotte or Lolita? We do know that as he writes his story, he is waiting to be judged and so takes every opportunity to slip in a defense. However, he dies before his trial begins.
By dying before his trial, Humbert never faces justice. It is unlikely that his memoir would have presented convincing defense material.
While Humbert's crime against Quilty is one of passion, his crime against Lolita is far more calculated.