The seven deadly sins of traditional Catholic theology are pride, lust, gluttony, greed, sloth, envy, and wrath. Given Beowulf's Christian themes, it might not be entirely surprising to find that the notion of sin (so critical to Christian thought and theology) is also a key component in the poem. Indeed, it is probably most powerfully expressed in the monsters Beowulf must fight and overcome, who seem to be physical manifestations of sin itself, opposing humanity at an almost existential level.
With this in mind, it is worth keeping in mind that these antagonists cannot be reduced to representing a single cardinal sin, any more than Beowulf himself can be reduced to a single virtue. Beowulf is highly courageous, and yet also humble before God. He is loyal to his liege and to his supporters, returning whatever loyalty he is shown. The same thematic complexity can be applied to a monster like Grendel. Just as the larger idea of virtue is far larger and more significant than the various individual virtues that a human being might exhibit, so is the larger idea of sin far more significant than the Seven Deadly Sins taken by themselves.
Grendel is the first monster Beowulf must face within the poem, and in his characterization, you can observe the presence of multiple sins. He is envious of the happiness enjoyed by the Danes, and this drives him to wrath as he attacks the Mead Hall. I think you can also identify Grendel with lust, given the sadistic pleasure he takes from his terrorizing of the Danes, as well as with gluttony, given the image of Grendel devouring human beings.
On the other hand, there is also Beowulf's last great opponent, faced at the end of his life: the dragon. Like Grendel, the dragon combines multiple sins within a singular characterization, and this combination is already present in the very image that serves as the dragon's introduction, with it asleep upon its treasure hoard. Note, in particular, the dragon's very passivity: before the theft, the dragon spent centuries content in its indolence. In this respect, it can be said to represent sloth. Yet at the same time, the dragon is also a powerful image of greed, given the treasure it hoards.