The confessional nature of Plath's "Daddy" makes the use of powerful imagery justifiable.
Plath writes "Daddy" from a confessional frame of reference. The interactions Plath had with her father had a profound impact on her identity. They were difficult to understand, and her poetry is a way for her to make sense of it.
Plath sees her father as dominating. She feels he denied her voice and expression. He was a "black shoe" in which Plath "lived like a a foot," afraid to "breathe" or sneeze. Plath writes how she had "always been scared" of her father. Her trepidation in communicating with him is seen in lines such as "the tongue stuck in my jaw." It conveys the fear that prevented a daughter from being able to speak to her father.
The use of "Hitler" communicates this emotional dynamic. Plath compares herself to someone who is Jewish living at the time of concentration camps. In likening herself to a Nazi victim, Plath conveys the extent to which her father denied her voice. He "killed" off a part of her identity. Plath's father made her believe she "may well be a Jew." Plath's argument that her father robbed a portion of her identity makes the Hitler comparison fitting, if extreme. What Hitler did on a political level to millions of people is what Plath feels her father did to her on a personal level.
Plath feels her father took something from her. He took her happiness. His dominance over her was similar to how Dracula wielded power over his victims. Both malevolent forces take the lifeblood of their victims. Her call to drive a "stake in your fat black heart" reflects how Plath wants to restore power. Just as Dracula has to be killed to make people feel safe, Plath believes she must do the same to her "vampire" of a father. In "Daddy," the relationship between Plath and her father makes the images of Dracula and Hitler highly effective.