The concept of negotiating what makes us human through looking at representations of human beings with grotesque tendencies is brilliant. These characters – like Hannibal Lecter – are certainly monsters in a psychological sense.
To say that they are more powerful than monsters for the purposes of negotiating our humanness, in a narrative, might be taking it too far – I would say they are a powerful example, but that they only fulfil another type of monster in the repertoire of monsters.
The intelligent, alive, and psychologically disturbed human-monster (or inhumane-human) represents the monstrous potential within a person extremely well; the Other is almost as close as it gets to us. On an intrapersonal level, this type of monster is extremely effective, it indeed resonates deeply.
Outside of this, vampires look pretty human-like, zombies less so, and ghosts aren’t even physical [Aside: I think the black smoke monster (or The Man in Black) from LOST would prove to be an interesting artefact: ghost-like and physical (morphing), immortal and supernatural, immune to death, and longing for freedom (that ultimately leads to an unwanted death)]. But, my point is that though human-monsters are an effective example, the vampire and zombie (ghost and mummy) lead us to question our humanity in different ways.
Zombies, more specifically zombie apocalypses (apocali?), like in The Walking Dead, lead us to question our humanity: will we retain our humanity in a fight for survival? how will people relate to one another in dire circumstance? will we retain a sanctity of life? what is the purpose of life? etc.
The inhuman-human, human-monster, or human being with grotesque tendencies, whatever it be called, is a welcome addition to the list of monsters.