Theme of envy: As a descendent of Cain, Grendel is the biblical son of Adam and Eve, who out of jealousy kills his brother Abel (Genesis 4). He is eternally doomed. Grendel's response in envy to Heorot's light filled and happy celebrations. The scops "Song of Creation" reminds him of the loss he suffered because of Cain's sin and makes him angry and seek for revenge from him stemming envy.
Theme of revenge: The central theme of revenge serves as motivation to many characters. Grendel delights in revenging Heorot as he hates about other men success, glory, joy and favor in the eyes of god.
Grendel's mother avenges Heorot against her son's death, motivated by mother's fury.
The dragons embark revenge against fugitive slave for theft from his hoard of treasure by raiding the countryside and burning the Beowulf's home to which Beowulf seeks revenge against dragons for the destruction caused.
