Inwardly, Grendel had suffered his whole life. Does Grendel feel joy in defeat when he is bested by Beowulf? There may be joy in Grendel's heart because this ended his struggle with the two beliefs he encounters in the novel. During his death, Grendel has a revelation he should have had while still alive.
Exactly how do we know?
All who hear Grendel shriek in fear and pain are frightened. As Beowulf fights with Grendel, Beowulf's men heroically hack at the demon, but no weapon on earth can harm him.
It is important because a narrative is supposed to tell you about all the important parts of a story so without the key events it wouldn't be a complete narrative.
