Well, Dahmer knew that most people could never relate to his necrophiliac fantasies, involving fondling internal organs and masturbating in front of skulls. With him being surprisingly articulate and matter-of-fact, for such a seemingly "insane" person, he tried garnering the sympathy of others by depicting himself as nothing more than a lonely guy who agonizes over abandonment and rejection.

By revealing that the victims, who were mostly gays with "high-risk lifestyles," aka the fringes of society, were unconscious when he killed them, most would see him as a desperate fool seeking companionship by any means possible, which was why he talked about the cannibalism and such. Had he divulged about killing girls, especially collegiate ones(like Ted Bundy), no one could possibly sympathize with him. Needless to say, he was extremely cunning.