The confrontation between Grendel and Hrothgar, in the epic of Beowulf, exists because of Hrothgar's loyalty to God (today's version is presented from a Christian perspective). As defined by the text, Grendel has been exiled from the light by God because of his relation to Cain. Since exiled for another's act, Grendel defines God as his enemy. Since unable to enact his revenge upon God himself, Grendel chooses to enact his revenge upon those who worship God.
Hrothgar's love for God leads him to build a master mead hall, in order to give praise to God for his (Hrothgar's) victories. The people celebrating and praising God in Heorot (Hrothgar's mead hall) enrage Grendel. In an attempt to stop the praise of God, Grendel attacks Heorot so many times that Hrothgar is forced to close its doors and cease the praise of God.
Therefore, the symbolic meaning behind the confrontation between Grendel and Hrothgar lies in the continual battle between good and evil, light and dark, and Christian and pagan. Grendel symbolizes evil, darkness, and pagan life. Hrothgar symbolizes good, light, and Christianity. As natural "enemies," the symbolism lies in the eternal conflict between the good and evil in the world.