The monster in Beowulf is called Grendel. Grendel has been exiled by God for being a descendant of Cain (the original murderer of the world). Since it was the actions of his ancestors which forced his exile, Grendel is not accepting of anyone who is able to live within God's light.
Since Heorot was built by Hrothgar to celebrate God, only those who are welcome in God's light are able to take part in the celebrations at the mead hall. Since exiled, Grendel is not allowed to approach the throne of God (within the walls of Heorot).
The Danes, celebrating life and God's power, provoke Grendel's wrath. The constant celebrations by the Danes tormented Grendel. Since he was unable to celebrate in God's light, the constant reminder of his exile proved to be far more than he could take. Grendel could only enact his anger against the mead hall and those who celebrated within its walls.