Envy and revenge are both central themes to Beowulf, and they're both addressed in numerous ways.Envy: The clearest example of envy can be seen in Grendel's response to Heorot's happy, rowdy, and light-filled celebrations. As a descendent of Cain, Grendel is an outcast of organized society, and he's condemned to wander in a lonesome fashion. As such, the fact that others enjoy community and friendship makes Grendel not only angry, but envious. Thus, we can see Grendel's attacks against Heorot as a display of his envy.Revenge: Revenge is central to the warrior culture described in Beowulf. In particular, revenge was usually taken through what was called a "blood-feud," which involved warriors fighting against those who had killed their kinsman, lord, or king. We can see revenge at work in many ways in the poem. For example, Grendel's mother avenges her son's death by attacking Heorot, while the dragon embarks on a rampage to avenge the theft from his hoard of treasure (and then Beowulf accordingly fights the dragon to take revenge for the destruction). As such, we can see that revenge was an important element in the society described in the poem, and that individuals were honor-bound to retaliate against those who wronged them.