Why does Beowulf use Hrunting to attack Grendel's mother?

Answer 1

Beowulf tries using Hrunting, the sword the Danes had given him, to attack Grendel's mother because the mother wanted to avenge her son's death and was followed into her home in the lake by Beowulf. Beowulf's weapon was Hrunting and so he tried attacking the mom because of her previous actions (avenging the death of her son Grendel- the monster Beowulf killed- by killing a close friend of the king Hrothgar who's men were killed by Grendel, and retrieving Grendel's arm that hung from the rafters of Herot after the battle when Beowulf hung it there from tearing it off Grendel).

Answer 2

Beowulf uses Hrunting to attack Grendel's mother because it is a sword that he was given by Unferth, a warrior in Hrothgar's court. Despite Hrunting being a well-crafted weapon, it ultimately fails to penetrate Grendel's mother's skin, leading Beowulf to later rely on his own strength and a magical sword found in her lair to defeat her.