Grendel’s mother avenges her son by attacking Heorot at night, killing Hrothgar’s friend Aeschere, and taking with her the trophy of her son’s dismembered arm. Unlike Grendel, the mother’s motivation seems purely retaliatory; she is less interested in plunder than she is in making the Danes pay. This is why she leaves such a clear trail behind her when she returns to her lair in the lake, and why she marks the spot for Hrothgar and Beowulf with Aeschere’s severed head. She is luring Beowulf into the lake, where she hopes to have a final revenge. Beowulf is more than up to the challenge, however. He jumps in the lake, and is right away captured by Grendel’s mother, who takes him to her underwater home. There is a savage fight, during which the sword Unferth gave Beowulf fails; luckily there is a huge sword, meant for giants, that is part of the mother’s armory, which which Beowulf finally kills her.
The death of Grendel’s mother is one of the most violent parts of the poem. Unlike Beowulf, who seeks glory, Grendel’s mother only seeks vengeance for her child. After Beowulf kills her, he uses the giant sword to decapitate Grendel's corpse, which is in his mother's lair, and brings the head back to Heorot as a trophy. One can see how a reader might sympathize with the grieving monster in this situation.