The concepts of good and evil in Beowulf are very important. Identified as a major theme within the text, Beowulf and his epic battles depict the historical war between those who war for God (Beowulf) being good and those who oppose God (Grendel, his mother, and the dragon) as evil.
In the epic, the protagonist fights a God-despiser. In Beowulf, Grendel and his mother represent God-despisers. Since being exiled for being a kin of Cain, Grendel is jealous of those who are able to revel in the light of God. This sets him up to be a natural antagonist of Beowulf (a warrior for God). In both his fight with Grendel and his mother, Beowulf recognizes that God is ultimately in charge. He, Beowulf, does not fear death because if his death is God's will he will openly welcome his own death.
In the end, the battles are all about good (Beowulf) and evil (the monsters) and Beowulf's triumph symbolizing the victory of good over evil. Essentially, Beowulf's battles with both Grendel and Grendel's mother present the concept of good and evil.