Both Macbeth and the Creature in Frankenstein become "monsters" as the story progresses. All three are considered as going against nature in one way or another.
Grendel is an outcast who is considered unnatural from birth. Frankenstein wants to be loved. He is rejected by his father, who sees him as hideous. He is also considered unnatural from the moment he comes to life (although in Frankenstein's defence, his father created him out of dead criminal's body parts, so one has to wonder what the man expected his "son" would turn out like). Macbeth, on the other hand, starts off as an exemplary character and then declines. He is not a product of his environment as much as a product of his own ambition, and his willingness to trust his supernatural guides. He becomes a monster after he gives into his base desires.
All three end up tormenting the society that they live in the fringes of. Grendel and Frankenstien are never accepted; Macbeth, though he becomes king, ostracises himself by his actions.