Why does Hrothgar ask Beowulf to battle Grendel's mother?

Hrothgar asked Beowulf to battle Grendel's mother because the king believed he was the only man capable of dealing with such monsters. This was because Beowulf had earlier succeeded in mortally wounding Grendel. Further, Hrothgar wanted revenge for his close friend.

“the first decision that led to brutus’ downfall is when he decided that he would join the conspirators in their plot to kill caesar. the plot did not work. the conspirators had to flee for their very lives. the second decision that led to brutus’ downfall is when he refused to listen to cassius. “ i hope this information you.