Irony is discrepancy between expectation and reality. In Beowulf, what is ironic about the way Beowulf kills Grendel's mother?
As stated in the question, irony is the discrepancy between expectation and reality. In Beowulf, during the battle between Beowulf and Grendel's mother, some readers may find how Beowulf kills Grendel's mother ironic.
As a true epic hero, Beowulf must live up to the promises (or battle boasts) he makes. After killing Grendel and Grendel''s mother kills Aeschere, Beowulf promises Hrothgar that he will kill Grendel's mother as well. Unferth, wanting to make amends for challenging Beowulf's honor earlier, presents Beowulf with Hrunting (a sword). Hrunting is an heirloom weapon of Unferth's family. It has never failed the warrior who wielded it.
As Beowulf begins to fight Grendel's mother, he realizes that Hrunting is useless against her. Throwing the weapon aside, Beowulf spots a sword nearby. Grabbing the sword (made by giants), Beowulf brings the blade down upon Grendel's mother's neck and kills her.
The irony in the battle with Grendel's mother lies in the fact that she is killed by her own sword. The man-made sword (Hrunting) would not harm her, yet the sword of the giant's, one which was magic, succeeded at killing the monster. Magic was killed by magic.
What is ironic about the way Grendel's mother is killed in the story Beowulf, is that she was killed with her own sword. Grendel's mother was trying to avenge Grendel's death who was maimed by Beowolf in a struggle between the two. So although she was trying to avenge Grendel's death, she is killed by Beowolf as well.
Beowulf is a noble warrior he wouldn't kill a women just the enemy also he saw that that Grendel's mother was tryimg to avenge his death so he killed her.