Beowulf Death-Price Poem

What the reasearch paper is about: In Beowulf, much is made of paying the "death-price" (line 156). To help with your understanding, see the accompanying footnote to this line. Grendel never did pay for the men he killed, and so Beowulf's killing of Grendel is justified in this culture. However, Grendel's mother avenges her son's murder, which in this culture is justified. What, in the eyes of Anglo-Saxon culture, makes Grendel's mother's act so monstrous if she was only doing what is expected? In other words, why is her act seen as evil while Beowulf's is not?