How does the contrast between Grendel and Beowulf turn their conflict into a fight between good and evil?
Grendel is a life-taker. He comes from a line of killers, starting, of course, with Cain. Cain was the first killer in the Bible; he killed his own brother Abel. Cain killed out of jealousy. In terms of good and evil, we often define evil as anything that destroys or diminishes life for selfish purposes. Grendel, like Cain, kills for selfish purposes. He is jealous of the joy being experienced by the humans, and he kills because he is annoyed. By contrast, Beowulf is a life-protector - a life-saver. He upholds the beliefs and traditions of his society; in this respect, he builds on his society, growing it. He comes as a savior to help the people being ravaged by Grendel. He comes to save the people from the most ancient of evils - murder. This creates a dichotomy of good verses evil, life-taker verses life-protector.