One of the most striking features of the narrative (you'll figure out which book I'm discussing here, so I won't tell you) is that it is the mother who attempts to avenge Grendel - not Grendel's friend, brother, or father. Why the mother?
One answer might lie in the bizarre mix of paganism and early Christianity which characterizes this book; this uncomfortable blend is also found in the Nibelungenlied (a famous Germanic folktale), and in the ideas surrounding the earliest formulations of chivalry and courtly love.
Did the author intend to see in Grendel and his mother an inverse image of Christianity? Is Grendel an anti-Jesus figure? If so, then his mother would be an anti-Mary, hence her otherwise unexpected prominence in the narrative.
Can we find other hints that Grendel might be mirror image of Christ? Grendel crawls into his grave to die, instead of emerging from it to live. Grendel brings fear and death, instead of "peace be with you" and life.
Perhaps the author was aware of his clashing mix of violent paganism and altruistic Christianity, and so wove into his story an opposite analogue to a Christ-figure. Is there then also a Christ-figure in the story? You decide.
