King Hrothgar is a mentor to Beowulf. Beowulf doesn't really have many people so Hrothgar acts as a mentor. King Hrothgar actually knew Beowulfs father. So in a way Beowulf feels he owes him a simple favor of defeating grendel for th king.
Beowulf vs Grendel
Beowulf goes to battle Grendel. After Grendel breaks into Herot to attack and kill and destroy Herot.
The End of Grendel
Beowulf witnesses Grendel attacking and killing a geatish warrior. Beowulf observes this and when Grendel notices him he will attack him.
Grendel is surprised when Beowulf unleashes his strength on him. Grendel is scared and feels Beowulf could win. Eventually Beowulf does overpower Grendel and rips off his arm.
Grendel after getting his arm ripped off by Beowulf runs away to his swamp home before Beowulf could do anymore to him. Grendel dies alone and in the swamp
King Hrothgar congratulates Beowulf on his victory of defeating Grendel and thanks him for his bravery. Beowulf shows the King his trophy, the arm of Grendel.