Beowulf is an epic poem which was first created in the oral tradition centuries ago. Though there are many elements of the story which are not applicable to modern life or readers (such as dragons and superhuman feats), readers are still able to connect to the themes and sentiments of the narrative. Grendel's mother is a good example of a recognizable character despite the fact that she is a
Devil-shaped woman, her woe ever minded,Who was held to inhabit the horrible waters.
First of all, Grendel's mother is, of course, a mother. While she is certainly not a typical mother in appearance or in the offspring she produces, Grendel's mother has the same instincts as any modern mother. When her child has been killed, she grieves for him. Grendel left the mead hall and went to his mother to die; the next day, she goes to Heorot to retrieve her son's arm. While the image and the act are both gruesome, her desire to mourn her son and keep his body intact even after death is a universal emotion.
Second, Grendel's mother seeks revenge for her child's death. It is true that she goes to the extreme (kidnaping and killing a man), but the desire to have have someone pay for the untimely death of a child is understandable and even shared by many women (parents) today.
Third, Grendel's mother is willing to fight on behalf of her son. Though Grendel is gone, when Beowulf comes to her underwater den, she fights--and nearly wins--to keep her child's body from being further mutilated or destroyed. She must know that Beowulf is going to use Grendel's dead body as some kind of a trophy, and he does. No mother would want her child to be used in such a way and would fight however she must to prevent it.