The epic Beowulf exists as a text which portrays the ever present and consistent battle of good and evil. One of the evil beings in the text is introduced in the opening chapter of the epic. Grendel, a descendant of Cain, has been exiled by God (from his light) because of his ancestor. Given that Grendel took no true part in his banishment, he hates both God and those who worship God.
Hrothgar built Heorot to honor all that the Lord had blessed him with (being power, wealth, and success in battle). Given that Grendel was "tormented by the hall's jubilant revel day by day," his anger and hatred for the Dane people grew immensely. Not only did Grendel listen to the praise of God, he had to hear the people praise God for creating a beautiful world, filled with light, which they could worship in. Given his exile from God's light, Grendel hated what Heorot represented--God and his love. Since Grendel could not act out against God himself, he choose to act out against God's people and the place built for the worship of God, Heorot.
Although the current, animated film "Beowulf" presents Hrothgar as Grendel's father, the book does not. The symbolic meaning of the confrontation between Grendel and Hrothgar is one stemming from a universal theme: the struggle between good and evil. The book includes references to Grendel being of the family of Cain, noted in the Bible as the first murderer and child of Satan, i.e. evil, in contrast to the respected and lauded king of the Danes.
I hope this has been helpful.