There are two ways to describe the "dark side" that Grendel represents. Many critics see Grendel as representing fear and cowardice in the poem. In Anglo-Saxon literature, this would be a "dark side" in a warrior who could not defeat his enemy. After all, Grendel's rampage lasts for 12 years. In addition, Grendel kills without mercy and in the dark of night. This would also symbolize a "dark side" for a warrior who thinks of himself as morally upright and selfless. When Beowulf kills Grendel, he is defeating both the fear and cowardice the men have felt in their unsuccessful battle against Grendal. In addition, Beowulf not only confronts Grendal in the dark and mortally wounds him, but then he follows Grendal to his lair and finishes him off. Beowulf faces his enemy instead of using stealth under cover of darkness. By facing his enemy, Beowulf has overcome the darkness of battle that Grendel brought.
It can be described as the defeat of the dark side because Grendel was a descendant of Cain (from the bible) who was the first murderer. He murdered his brother Abel. Grendel's Cain, has murdered many people and therefore is known for being evil.