There are many similarities between Grendel and Cain. First, it is stated that after Cain killed his brother, he drinks his blood. (Williams, 45) This is typically something that Grendel does after he kills his victims. Cain and his descendants were believed to have magical power, such as being able to shift their shapes and to have bright shining eyes. Grendel also has these magical powers. He is able to cast a spell on the warriors' swords, so that they cannot harm him.
The most significant similarity between Grendel and Cain, is that they are both outcasts of society and have to roam in the shadows. They are outside looking inside. They are outside threats to the order of society as shown by Grendel with the Danes. In Beowulf, the mead hall, Heorot, is the symbol of peace. It is the place where the warriors gather in a spirit of brotherhood and harmony to celebrate. When Grendel is first introduced in the story, the audience is told that he is enraged at the festivities and the sound of laughter that he is hearing while he is alone in his mere.
Therefore, Grendel represents "a monstrous outsider enraged by the joy of brotherhood and society from which he is forever banished. His enmity towards Heorot is grounded solely in this moral perversion, which is another example of the hatred of the good simply because it is good." (Williams, 45.) Grendel exhibits his envy towards the warriors as Cain did to his brother, Abel, so long ago.