What is Humbert's major defense?

Humbert Humbert basically claims insanity. He offers several defenses or rationalizations, both for the murder of Clare Quilty and for his behavior toward Lolita. The murder seems almost incidental in his manner of thinking, and it occupies less space in his narrative. Jealousy and frustration because of Quilty’s involvement with Lolita enter into his explanation of the murder. Casting doubt that, in his diminished state, he would have had enough energy to kill, he promotes the idea that he could have done so only in a “spell of insanity.”
Drawn to the girl by her innocence and purity, but also claiming that she exerts some allure, Humbert alternates between admitting to his desire and trying to convince himself and the reader that his interest in Lolita was not sexual. Here, too, he presents himself as if under a spell, hopeless to resist her. In defining the category of “nymphet,” for example, he says these maidens “reveal their true nature” to “certain bewitched travelers,” or men at least twice their age, over whom they have “fantastic power.”
Calling himself “an artist and a madman, a creature of infinite melancholy,” Humbert raises the possibility that insanity makes him irresponsible. In the same vein, he tries to minimize his behavior toward Lolita by comparing it to the greater crime and associating it with his creativity, noting that, as a poet, he is not a killer. Yet he also acknowledges that all his attempts to depict Lolita have failed utterly; however, although his approximation is fanciful, to him it is “perhaps, more real than Lolita.”
Ultimately, of course, he is a killer. He not only pushes off the blame for pedophilia onto the girl he dreamed of violating, he also further rationalizes the murder by blaming it on his unrequited desire for her. “Taboos strangled me,” he says, rather than admitting to physically harming actual persons.